It is a server side bug. So a change that Geocaching HQ made to their API (not sure what) is now triggering the 500 error when full cache data and 100 caches are requested.
Do you have the "Full Cache Data" option turned on? If so, try turning that off and see if that helps.
If it DID fix the issue, can you let me know what you have for the number of caches to load setting? Is it 100?