user450: I have encountered a probable bug in Premiere Elements 10, and fortunately I've also found a workaround which is why I wrote my own answer below. But since I haven't been able to solve the underlying problem, I'd be very glad for additional ideas on how to fix the problem. Now here's the question:
How can I salvage a Premiere Elements project file (.prel) that can't be opened because of the following error:
Premiere Elements Debug Event
Premiere Elements has encountered an error:
[d:\pre\mediacore\mediafoundation\api\inc\Keyframe/Keyframe.h-142]

This error appears to occur sporadically on non-US versions of Windows. There is no previous indication during the editing session that anything may be wrong (saves and auto-saves work fine), but on the next restart, the project can't be opened. Other projects appear to be unaffected. 
According to other users who have seen the same error (Google for "premiere debug keyframe" and you'll find some), there might be some connection with the introduction of DVD menus into the project, but I'm just speculating here.

user2670: 1. Recover broken files
I found this online application very helpful.
http://www.vibait.com/software/premiere-fix-keyframe.h-142/
Simply upload your Premiere Elements file and you get a fixed version in return. Save it to your disk and open it.
With one file I got error messages like this one a few times:
Parse error on line 1431. Expected 8 values, got 9 
Old value: 116050644638,87,5,0,0,25,791250419642971,0,16666666666666666,1,2731148504048133,0,16666666666666666 
New value: 116050644638,87,5,0,0,25.791250419642971,0.16666666666666666,1.2731148504048133,0.16666666666666666
Using an editor I searched for some of the mentioned numbers in the XML file and simply cleared the XML element (deleted all numbers).
2. Fix it (i.e. a workaound)
A few web pages mentioned that the number formats of the regional settings (especially , and .) may be involved in this problem. After setting "." as the decimal separator I did not have this problem again.

user450: Analyzing the files that fail to open in contrast to those that do open just fine shows one glaring problem: All lines in the project.prel file (which is just an XML file) where there should be decimal numbers in <StartKeyframe> or <Keyframe> tags are damaged. Example: 
Correct version:
<StartKeyframe>-91445760000000000,100.,0,0,0.,0.,0.,0.</StartKeyframe>

Defective version:
<StartKeyframe>-91445760000000000,100,,0,0,0,,0,,0,,0,</StartKeyframe>

So apparently Premiere Elements at some point got confused by European locale settings and decided to save decimal values using the European decimal separator (comma) instead of the US decimal separator (dot). Of course it's then impossible to open that file correctly ever again.
Fortunately, at least in my tests, there are only three kinds of decimal values in .prel files:

integer floats: 1., 0., 100.
the single value 0.5 
double precision floats: 0.16666666666666666, 1.000000000000000000000000

This makes it possible to fix the file using a regular expression replace. If your editor supports Perl-style regular expressions, you can search for
,(?:(?=[,<]|\d{10})|(?<=\b0,)(?=5\b))

and replace all with
.

This will replace all erroneous decimal commas (i. e. those that are either followed by another comma, an opening angle bracket, a decimal fraction of at least 10 digits, or the number 5 (but only if that comma is preceded by the single digit 0)) with a decimal point.
After that (be sure to back up your .prel file before you do this!), the file can be opened again correctly. However, as soon as it's saved again, the error is re-introduced.
Therefore, if someone has an idea how to actually fix this problem instead of patching it up with an admittedly wonky regex, I'd be most grateful.

One hint as to what the underlying problem might be: Around the time the error was introduced, Premiere spontaneously opened a little dialog window that said (in German) something like
"Premiere Elements is updating a component. Information is being transmitted..."

While this window was displayed, two Win7 UAC boxes popped up, asking me to allow the Windows Regserver to update my system. The actual commands it needed approval for were
C:\Windows\System32\regsvr32.exe /s 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\mc_dec_mpa_ds.ax" 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\mc_dec_mp2v_ds.ax"

and
C:\Windows\SysWOW64\regsvr32.exe /s 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\32\mc_dec_mpa_ds.ax" 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\32\mc_dec_mp2v_ds.ax"

(line breaks inserted for clarity). 
The first time around, I allowed these actions (which in retrospect may have been a bad idea...); now this happens every time I'm trying to open a "patched" project file. It doesn't matter whether I now allow or refuse it - either way, Premiere writes a corrupted project file when I save my project.

450: <p>I have encountered a probable bug in Premiere Elements 10, and fortunately I've also found a workaround which is why I wrote my own answer below. But since I haven't been able to solve the underlying problem, I'd be very glad for additional ideas on how to fix the problem. Now here's the question:</p>

<p>How can I salvage a Premiere Elements project file (.prel) that can't be opened because of the following error:</p>

<pre><code>Premiere Elements Debug Event
Premiere Elements has encountered an error:
[d:\pre\mediacore\mediafoundation\api\inc\Keyframe/Keyframe.h-142]
</code></pre>

<p>This error appears to occur sporadically on non-US versions of Windows. There is no previous indication during the editing session that anything may be wrong (saves and auto-saves work fine), but on the next restart, the project can't be opened. Other projects appear to be unaffected. </p>

<p>According to other users who have seen the same error (Google for "premiere debug keyframe" and you'll find some), there might be some connection with the introduction of DVD menus into the project, but I'm just speculating here.</p>
 2670: <p><strong>1. Recover broken files</strong></p>

<p>I found this online application very helpful.</p>

<p><a href="http://www.vibait.com/software/premiere-fix-keyframe.h-142/" rel="nofollow">http://www.vibait.com/software/premiere-fix-keyframe.h-142/</a></p>

<p>Simply upload your Premiere Elements file and you get a fixed version in return. Save it to your disk and open it.</p>

<p>With one file I got error messages like this one a few times:</p>

<p>Parse error on line 1431. Expected 8 values, got 9 </p>

<p>Old value: 116050644638,87,5,0,0,25,791250419642971,0,16666666666666666,1,2731148504048133,0,16666666666666666 </p>

<p>New value: 116050644638,87,5,0,0,25.791250419642971,0.16666666666666666,1.2731148504048133,0.16666666666666666</p>

<p>Using an editor I searched for some of the mentioned numbers in the XML file and simply cleared the XML element (deleted all numbers).</p>

<p><strong>2. Fix it (i.e. a workaound)</strong></p>

<p>A few web pages mentioned that the number formats of the regional settings (especially , and .) may be involved in this problem. After setting "." as the decimal separator I did not have this problem again.</p>
 450: <p>Analyzing the files that fail to open in contrast to those that do open just fine shows one glaring problem: All lines in the <code>project.prel</code> file (which is just an XML file) where there should be decimal numbers in <code>&lt;StartKeyframe&gt;</code> or <code>&lt;Keyframe&gt;</code> tags are damaged. Example: </p>

<p>Correct version:</p>

<pre><code>&lt;StartKeyframe&gt;-91445760000000000,100.,0,0,0.,0.,0.,0.&lt;/StartKeyframe&gt;
</code></pre>

<p>Defective version:</p>

<pre><code>&lt;StartKeyframe&gt;-91445760000000000,100,,0,0,0,,0,,0,,0,&lt;/StartKeyframe&gt;
</code></pre>

<p>So apparently Premiere Elements at some point got confused by European locale settings and decided to save decimal values using the European decimal separator (comma) instead of the US decimal separator (dot). Of course it's then impossible to open that file correctly ever again.</p>

<p>Fortunately, at least in my tests, there are only three kinds of decimal values in <code>.prel</code> files:</p>

<ol>
<li>integer floats: <code>1.</code>, <code>0.</code>, <code>100.</code></li>
<li>the single value <code>0.5</code> </li>
<li>double precision floats: <code>0.16666666666666666</code>, <code>1.000000000000000000000000</code></li>
</ol>

<p>This makes it possible to fix the file using a regular expression replace. If your editor supports Perl-style regular expressions, you can search for</p>

<pre><code>,(?:(?=[,&lt;]|\d{10})|(?&lt;=\b0,)(?=5\b))
</code></pre>

<p>and replace all with</p>

<pre><code>.
</code></pre>

<p>This will replace all erroneous decimal commas (i. e. those that are either followed by another comma, an opening angle bracket, a decimal fraction of at least 10 digits, or the number 5 (but only if that comma is preceded by the single digit 0)) with a decimal point.</p>

<p>After that (be sure to back up your <code>.prel</code> file before you do this!), the file can be opened again correctly. However, as soon as it's saved again, the error is re-introduced.</p>

<p>Therefore, if someone has an idea how to actually fix this problem instead of patching it up with an admittedly wonky regex, I'd be most grateful.</p>

<hr>

<p>One hint as to what the underlying problem might be: Around the time the error was introduced, Premiere spontaneously opened a little dialog window that said (in German) something like</p>

<pre><code>"Premiere Elements is updating a component. Information is being transmitted..."
</code></pre>

<p>While this window was displayed, two Win7 UAC boxes popped up, asking me to allow the Windows Regserver to update my system. The actual commands it needed approval for were</p>

<pre><code>C:\Windows\System32\regsvr32.exe /s 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\mc_dec_mpa_ds.ax" 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\mc_dec_mp2v_ds.ax"
</code></pre>

<p>and</p>

<pre><code>C:\Windows\SysWOW64\regsvr32.exe /s 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\32\mc_dec_mpa_ds.ax" 
   "C:\Program Files\Adobe\Adobe Premiere Elements 10\32\mc_dec_mp2v_ds.ax"
</code></pre>

<p>(line breaks inserted for clarity). </p>

<p>The first time around, I allowed these actions (which in retrospect may have been a bad idea...); now this happens every time I'm trying to open a "patched" project file. It doesn't matter whether I now allow or refuse it - either way, Premiere writes a corrupted project file when I save my project.</p>
