Did you verify that the signature on your design elements still has the correct access rights on the server?

I have seen this frequently on a server running XPages applications where Notes Objects are not being recycled properly. You should be able to search the server log for the text JVMDUMP to see if a dump occurred somewhere before the Error 500 started occurring. The only immediate solution I found was to restart the HTTP task which frequently ended up crashing Domino server which then had to be restarted itself.

The real solution is to be extremely conscientious to recycle ALL Note Objects in your Javascript (and presumably java) code in all XPages applications.

I have had to go back over (when I first started developing XPages apps) all of my XPages, Custom Controls, Javascript libraries and agents and find every instance where a Notes Object was created and recycle it when I'm through with it. Then because I was on a shared server, I still had problems because of other developers' apps who, presumably, did not recycle objects.

It is absolutely critical to recycle all Notes Objects in your Javascript code.

I'm going to respectfully disagree with Rod. Just a different opinion really.  Yes technically it's very important to recycle Notes objects. Though I don't think you ever want/need to recycle Session and possibly also database. However my understanding is that in normal practice you only really need to worry about this inside loops.  You do need to be extra careful when you're doing ViewColumn stuff especially if there are dates in the column.  Any dates really since Dates come from the Session they can be tricky with a longer lifespan then you'd think.  When I was doing SSJS I never really ran into a recycle problem except in the case of loops.  These days I use Java and the OpenNTF API which handles all recycling automatically.  So it's not something I think a lot about anymore.  There should be some good information on recycling on Paul Withers blog.

Robyn - I would at least attempt to put the XPages Log File reader on your server.  Now that being an Xpages app might not run either but if it did run it would point more towards your home grown apps vs all apps.  And if it did run it would give you easier access to some of the log information.  That's all I can think of at the moment.  You might want to try this question on StackOverFlow.  Lot of top experts monitor those questions more closely and this is a good fit for that website.

Good Luck
Dave

Placed in C:\Lotus\Domino\jvm\lib\ext were the cause here... They were creating some sort of conflict with other JAR files that are referenced by XPages. The presence of these external JAR's caused any XPage to throw an error 500. We fixed this by moving the external JAR files to a folder outside of Domino. We then added a "JavaUserClassesExt=" line to notes.ini so the external JAR's could still be referenced by other Domino apps.