I'm getting another error 500, this time when I try to use the green back-and-forth buttons to move back one day, week, or month in the calendar. It's the same 'Error 500: OK' seen in other bug reports. I attempted to go into the code to apply the same sort of fix as prescribed for the other bugs, but I didn't have a clue what I was doing :-)
Switching to another week, month, or day can still be accomplished by using the 'Go To' date picker widget.
Oh, that's interesting! Today it's working fine, but yesterday (30 March) it wasn't. The only change I made was to add an event for 7 April.