I am also experiencing this issue with being unable to dismiss alarms. The error message I receive is MODIFICATION_FAILED and is consistent with what others have described here. I seem to be able to dismiss events without a problem as long as they do not have any attendees. Any events with attendees exhibit this problem in my testing.
I've attached the console output from Thunderbird during my tests of a successful dismissal, a failure, and a failure with MODIFICATION_ERROR. |