This is an upgrade from the latest version 4. I upgraded to 5.0.1, then 5.0.2 and finally 5.0.3. No errors during the upgrade process, and the forum seems to be working properly. The errors started to appear on March 11, between 3.19pm and 3.49pm. That's when I added an event to the calendar. No other apps installed, only Calendar. I've just run a test: when I add an event, the error doesn't reappear in the system log. Weird ๐