?corrupted iCloud calendar
I've been through similar issues (not the same issue ... but iCloud weirdness that I couldn't pin to any device).  Turns out it is possible to wedge iCloud even though all your local devices are ok.  

The fix:


Take backups of everything you can (your computer, phone, iPad, etc.)
Go to each device you own and log out of iCloud 


It is critically important you log EVERY device out of iCloud because the fix relies on you having zero devices that are logged in.


When NO devices are logged into iCloud, then (and only then) iCloud will dump it's cache for all data that it was syncing to your devices.  If even a single device remains connected to iCloud ... this will not happen.


Pick your most reliable device and log back into iCloud.  It will start syncing it's state (such as your calendar) to iCloud.

You can start re-reconnecting other devices back into iCloud one by one (I tend to do this very slowly to make sure each device has time to fully sync before I add the next device). 

If the problem turns out to be corrupted iCloud cache (and this has happened to me), then this should fix it.  If the problem is with a specific device then you may need further steps.