<p>Previously our SharePoint server was 32-bit and we used the web capture web part to display a bugzilla search results page. Since we've migrated to a 64-bit server the webpart no longer works. We're running the same versions of everything, the only change was moving from a Windows Server 2003 32-bit box to a Windows Server 2003 64-bit box.</p>

<p>Oddly enough, the logs don't contain anything. The pages where the webparts appear are the only place that error messages appear. Here is what I have:</p>

<p>-Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.</p>

<ul>
<li>Error - An unexpected error has been encountered in this Web Part.</li>
</ul>

<p>The reason I suspect the 64bit vs 32bit issue is that I saw numerous posts on the subject in various forums. It would appear that I am not the only one with this issue and all the troubleshooting has lead to this conclusion.</p>

Not strictly an answer to your question, but have you tried it using a sql server based session store? (Search on MSDN for the permanent script rather than the temp script that's provided with asp.net)
I've heard "bad things" about the executable session service, and consequently have not used it. Never had any problems web farming with the sql server based solution though.
Sorry it's not strictly an answer to your problem, but it should either (a) fix it, or (b) narrow it down significantly.