user5244: I started co-mining on Slush's pool successfully for about 48 hours, but from there on I haven't been able to connect to it again. It logs in the console this, no matter if I'm using a valid worker and password or not:
2013-05-26 12:55:19: Listener for "Default" started
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, checking for stratum...
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, started OpenCL miner on platform 0, device 0 (Turks)
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, no response to getwork, using as stratum
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20,  No JSON object could be decoded
2013-05-26 12:55:30: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:30, Failed to subscribe
2013-05-26 12:55:32: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:32, IO errors - 1, tolerance 2
2013-05-26 12:55:33: Listener for "Default" shutting down

Of course, I've checked my connection, and I've started GUIMiner with administrator privileges. I'm using Windows 7.

user5593: Use stratum.bitcoin.cz and port 3333 in GUIMiner, not like http://stratum.bitcoin.cz:3333 that was mensioned in another answer.

user2769: The problem is that in GUIMiner, the URL for Slush's pool is:

api2.bitcoin.cz:8332

That address has been deprecated, and it falls back to the GetWork protocol. From Slush's Pool news:

10.03.2013
  Default mining URL for Stratum is stratum.bitcoin.cz:3333. If you're still using api.bitcoin.cz, please fix your URL to prevent fallback to deprecated Getwork protocol.

In the GUIMiner interface, instead of choosing "Slush's Pool" from the dropdown menu, choose "other" and then enter
http://api.bitcoin.cz:8332
or
stratum.bitcoin.cz:3333

5244: <p>I started co-mining on <a href="https://mining.bitcoin.cz/stats/" rel="nofollow">Slush's pool</a> successfully for about 48 hours, but from there on I haven't been able to connect to it again. It logs in the console this, no matter if I'm using a valid worker and password or not:</p>

<pre><code>2013-05-26 12:55:19: Listener for "Default" started
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, checking for stratum...
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, started OpenCL miner on platform 0, device 0 (Turks)
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20, no response to getwork, using as stratum
2013-05-26 12:55:20: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:20,  No JSON object could be decoded
2013-05-26 12:55:30: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:30, Failed to subscribe
2013-05-26 12:55:32: Listener for "Default": api2.bitcoin.cz:8332 26/05/2013 12:55:32, IO errors - 1, tolerance 2
2013-05-26 12:55:33: Listener for "Default" shutting down
</code></pre>

<p>Of course, I've checked my connection, and I've started <a href="http://guiminer.org/" rel="nofollow">GUIMiner</a> with administrator privileges. I'm using Windows&nbsp;7.</p>
 5593: <p>Use <code>stratum.bitcoin.cz</code> and port <code>3333</code> in GUIMiner, <strong>not</strong> like <a href="http://stratum.bitcoin.cz:3333" rel="nofollow">http://stratum.bitcoin.cz:3333</a> that was mensioned in another answer.</p>
 2769: <p>The problem is that in GUIMiner, the URL for <strong>Slush's pool</strong> is:</p>

<blockquote>
  <p>api2.bitcoin.cz:8332</p>
</blockquote>

<p>That address has been deprecated, and it falls back to the <em>GetWork</em> protocol. From <a href="http://mining.bitcoin.cz/" rel="nofollow">Slush's Pool news</a>:</p>

<blockquote>
  <p>10.03.2013<br>
  Default mining URL for <em>Stratum</em> is <code>stratum.bitcoin.cz:3333</code>. If you're still using <code>api.bitcoin.cz</code>, please fix your URL to prevent fallback to deprecated <em>Getwork</em> protocol.</p>
</blockquote>

<p>In the GUIMiner interface, instead of choosing "Slush's Pool" from the dropdown menu, choose "other" and then enter</p>

<p><strong><a href="http://api.bitcoin.cz:8332" rel="nofollow">http://api.bitcoin.cz:8332</a></strong></p>

<p>or</p>

<p><strong>stratum.bitcoin.cz:3333</strong></p>
