Same problem here.
Also note:
1) I have the same problem on a 2nd Mac, with Calendar events in sync between the two machines via iCloud.
2) the trouble is only with entries to iCloud calendars. Entries to eXchange calendars are fine. But I need my iCloud calendars!
3) other discussion lists on this problem suggest to re-set the calendar Refresh settings (from Push to 1 Min, or from 1 Min to 5 Min). This did not help.
4) I noticed that whenever I try to make an entry in Calendar, the console shows the following error message:
16/06/15 08:30:07,150 Calendar[13612]: [com.apple.calendarui.log.locations] [Couldn't find current location with error Error Domain=CalLocationErrorDomain Code=1 "The operation couldn’t be completed. (CalLocationErrorDomain error 1.)"]
No idea what that means, no idea how to fix it.
Can you corroborate these observations? Anything has come up in the meantime that solved the problem for you.
thanks, wlfgng