Hi,
After doing a bit of research, I tend to agree with David. When I initially researched in the forums, there was an abundance of "I upgraded and now thumbnails don't work" related threads. There still are. I know that the reasons why each of us is experiencing the same problem likely vary slightly, but I'm a liitle confused why several of us are having "hosting" problems related to the same file, which is seemingly located exactly where it is supposed to be. The attached article below is from the knowledge base of my hosting provider. The two causes that I can attribute it to are either an issue with my htaccess file (in which case, I would kindly request help from one of the administrators) or with a php timeout. Please see the excerpt in the article
"If your PHP script makes external network connections, the connections may time out. If too many connections are attempted and time out, this will cause a "500 Internal Server Error."
Is it possible that too many connections to youtube are being attempted? I really don't know. That being said, I need help. The plugin doesn't work now and it did before. There has to be a very simple reason.
I think it's safe to rule out the "too many network connections" scenario. On your server, the call to ajaxEndpoint.php fails immediately. If it were a timeout issue, we would be waiting for at least 30 seconds. Besides, TubePress knows how to handle timeouts and can fail gracefully.
The good thing about HTTP 500 errors is that they are nearly always logged somewhere on your server. If you haven't already, I would contact MediaTemple and ask them why this URL is returning an HTTP 500 - they should be able to check the server logs on your behalf.
Feel free to post your .htaccess file (PM to me if it contains sensitive information) and I'll see if anything sticks out.
I apologize that your upgrade hasn't gone smoothly - to say the least! There are lots of ways that things can fail, and it's difficult for us to diagnose problems immediately since we don't have visibility into every server that hosts TubePress.