Thanks desertblues. The link points to a new reply window to an existing thread ("What is in your cup?").
I think I know the culprit though. This is one of the larger threads in our forum, with over 20000 posts! Most likely, you have your threads displayed in what is called "threaded" or "hybrid" mode, where a tree is shown along with every post (displaying the relationship each post has to the others). Unfortunately, this feature doesn't scale well, and because it is rarely used (and not enabled by default), it has not been added to the list of improvements. So if you visit a very large thread as the one you linked to, eventually our server will give you this 500 error message, basically telling you that it aborted the request to avoid memory overflows. The best thing to do is to switch the display back to linear mode by following these steps:
Please let me know if this is what has caused your problem and if changing your settings solved it.