This seems to happen fairly regularly, Evolution will be running along happily, then it will just lose its mind. It suddenly won't be able to deal with the calendar, messages disappear, can't connect to the server, and so on.
This trick seems to clear up the problems pretty well, with no loss of data that I've noticed:
sudo dpkg-reconfigure evolution
After doing this, I had to step through the account set up wizard, but my .evolution directory seems to be untouched (I made a backup first, just in case).