For mobile deviced related calendar problem, it is generally recommended to disable the mobile device temporarilly to see if the issue continues. It would help us isolate the fault from mobile device.
If the issue continues after we disable the mobilc devices, the problematic message might not be deleted correctly or it has been corrupted in the mailbox level. We may remove it via MFCMAPI tool.
Hope it is helpful.
Fiona Liao
TechNet Community Support
