CalendarAgent continuous error generation in MacOS 10.12.4
The possible problem is that database schema or some of database entries are incompatible with latest Calendar app. Or may be these entries are corrupted.

There are 2 possible solutions:


fix db which is tricky.
drop db - it's just cached data of your cloud accounts.


to do the 2nd option:


stop Calendar App (ctrl+q), then 
open ~/Library/Calendars and
remove Calendar Cache files (you may just trash them)
start Calendar App again.


You will need probably reactivate your calendar accounts.

To do so, open calendar app settings and choose desired accounts in accounts section