We seem to be having the same issue, only we're on Notes 8.0.1/.2
He will create (chair) a meeting. It shows up in his calendar as it should. The following day, the meeting is deleted from his calendar and he ends up receiving an error "invalid or nonexistent parent document" when trying to open up responses to the meeting because it is no longer found on his calendar.
There have been no recent changes to his computer.
I had him create a test calendar meeting. Sure enough, the next day, the test had disappeared from his calendar and says invalid document when he opens up a response