I have had the problem of Calender / Reminder not responding (simulating it did) on and off.
Recently my ID was changed on Exchange Server and the entire situation started all over again.
After a few hours of testing my old method as described in this thread, I found a solution that worked for me:
1 Delete in System Preferences>Mail, Contacts & Calenders and Calender>Preferences the Exchange ID for Calender (not for the Mail)
(I always had 2, one for Mail, Contacts and Notes and 1 for Calender and Reminders
2 Make sure that in System Preferences the Exchange ID is ONLY connected to Mail, Contacts and Notes (NOT to Calender)
3 In KeyFinder throw away all Exchange related logins
4 Reboot (not sure it is necessary)
5 Create in Calender>Preferences a new account, only dedicated to Calender & Reminders and give this a different name.
NOTE: afterwards this one is not shown under Preferences>Mail, Contacts & Calenders
6 Wait long (with me it was more than 2 hours, you see your Calender in grey 'updating' in top bar).
Then it worked
Note: I used the external connection under internal connection in the Calender>Preferences Exchange Account and nothing under External. In Server Path /EWS/Exchange.asmx
This seems to work. Good luck and give some Minus Points to those who developed the Calender App at Apple!