user-1: This is really annoying. I've heard a lot about how Apple have removed a lot of different codec uses including the most needed Quicktime RGB+Alpha one in After Effects. I can no longer import videos with an alpha channel into Premiere. I've tried ProRes 4444 but that didn't work either. All I get is "The importer reported a generic error."
I've tried restarting the project, restarting my computer, reinstalling premiere and so on. This is absolutely crucial to my business and must be fixed soon. Does anyone have a work around to this? 

user4426: Only Prores 4444 supports alpha channels, see the manual:

The Apple ProRes 4444 codec offers the utmost possible quality for
  4:4:4 sources and for workflows involving alpha channels.

It seems to work for me, using the Animation codec with "Millions of Colors+". I'm on a Mac using Adobe CC. Here are the steps I took:

Export your AE comp using Quicktime Animation codec with Millions of Colors+ selected.
Import the file into Premiere CC
Right click on your footage and select Modify > Interpret Footage
Select Conform Alpha PreMultiplication to: Premultiplied Alpha
Press OK
Drag your clip into the timeline

With this you should be able to drag another clip beneath it, and see it through your original clip, as shown below (Intepret Footage settings are also shown):
 

-1: <p>This is really annoying. I've heard a lot about how Apple have removed a lot of different codec uses including the most needed Quicktime RGB+Alpha one in After Effects. I can no longer import videos with an alpha channel into Premiere. I've tried ProRes 4444 but that didn't work either. All I get is "The importer reported a generic error."</p>

<p>I've tried restarting the project, restarting my computer, reinstalling premiere and so on. This is absolutely crucial to my business and must be fixed soon. Does anyone have a work around to this? </p>
 4426: <p><strong>Only Prores 4444 supports alpha channels</strong>, see the <a href="https://documentation.apple.com/en/finalcutpro/professionalformatsandworkflows/index.html#chapter=10%26section=2%26tasks=true" rel="nofollow noreferrer"><strong>manual</strong></a>:</p>

<blockquote>
  <p>The Apple ProRes 4444 codec offers the utmost possible quality for
  4:4:4 sources and for workflows involving alpha channels.</p>
</blockquote>

<p>It seems to work for me, using the <strong>Animation codec</strong> with "Millions of Colors+". I'm on a Mac using Adobe CC. Here are the steps I took:</p>

<ol>
<li>Export your AE comp using Quicktime Animation codec with Millions of Colors+ selected.</li>
<li>Import the file into Premiere CC</li>
<li>Right click on your footage and select <em>Modify > Interpret Footage</em></li>
<li>Select Conform Alpha PreMultiplication to: <strong>Premultiplied Alpha</strong></li>
<li>Press <strong>OK</strong></li>
<li>Drag your clip into the timeline</li>
</ol>

<p>With this you should be able to drag another clip beneath it, and see it through your original clip, as shown below (Intepret Footage settings are also shown):</p>

<p><img src="https://i.stack.imgur.com/0mGoT.png" alt="enter image description here"> </p>
