First off, this is not the same error that Internet Explorer users are encountering where they are reporting a blank page when trying to process an error message.
In my case, the comment is posted even though the 500 ISE is displayed.
Here is a link to my page, you can test it for yourself. Feel free to spam comments if you want to test, I'll delete them later.
Edit: it appears that if I disable both options under "E-mail me whenever" then it works fine. E-mail server problem?