I receive the same error message. I believe the error message (at least in my case) is related to the tags assigned to the event.
If I have an event that is tagged with somebody's username it will cause this error. If I remove all of the tags and save the event the error message goes away.
If I then go back and assign a series of arbitrary tags and update the event, there is still no problem, as long as none of the tags were also a user name registered in the system.
But, as soon as I introduce a tag that also happens to be a registered user name it will throw that error when attempting to view the calendar tab on the month that the event takes place.