Hello, and sorry for the delay. In response to your questions:
1) My understanding is that it's a Common User's mailbox, in that they had to grant permission to view their calendar originally
2) The sync error looks like it only occurs once for each recurring meeting, rather than once for each single recurring date (that is, until the next refresh)
3) I do not see any of the errors related to my own calendar - it only seems to come from other shared calendars
4) I've added all accounts that I can related to our company domain, and the issue persists for new additions (and I had previously wiped out the entire profile and regenerated, and the issue continued). In short, I don't have anyone else to add, and recent additions did not seem to resolve the issue.
Related to the test account via your PM, I'm happy to perform the test if it helps you to troubleshoot, but since the issue happens only to certain recurring meetings, It's likely that the above test would look okay on my end, particularly if your account has an entirely different permission set than our internal account has. If that makes sense.
One thing that I've noticed, is that I have errors for events that are long past due. I'm not sure of this, but some may or may not have predated our switch to Outlook 365. Or possibly not, but some seem fairly old, and this issue may just relate to very old recurring invites. This may be a red herring though, there could be some new invites in here as well. The issue is, I can't request that my coworkers delete and recreate these extremely old events (there are hundreds).
The key thing for me is that this just started happening with the current build: 150113. Nothing else changed in-between related to my inbox or our security or configuration, and these errors definitely weren't there before I upgraded the build. They appeared directly after installing the latest version.
Thoughts? Thanks.