I had same problem re “default object” error messages. Found a solution after lots of googling…
Error seems to be caused by:
$evt->eventid = 0;
It occurs in 3 places in the WP-Calendar plugin (version 1.5.3): twice in fsCalendarAdmin.php; once in fsCalendarEvent.php
Solution:
Add this line in the .PHP files just above the problematic code:
$evt = new stdClass();
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Caveat emptor: I am not a programmer, and know just enough about code-hacking to be dangerous.
As for the Category problem, I’m experiencing that too. My workaround is to add them in the Post Categories area & advise my WP clients to do the same.
Hopefully these bugs will be fixed in the next version of WP Calendar.