I agree that it happens related to Shared Calendars (see some of my earlier posts). I have significantly reduced the number of shared calendars I access and I correspondingly get less of these errors.
Once the error happens, I usually have an email stuck in Outbox. I go into that message again and hit Send repeatedly until eventually it clears.
i have no idea what is causing the root error but do not want to invest my time with Microsoft support. If anyone gets more insight, please post!!