So helping out few people, all on Mac (latest version) along with the latest version of Outlook.
All is fine apart from a shared calendar they have access to, they are unable to cancel their own meeting they enter in this particular shared calendar, the cancellations get stuck in Outbox.
They are part of a security group which has PublishingAuthor rights, and I used one of their accounts on a Windows laptop to test it out, I am able to cancel the meetings entered on this shared calendar on Windows Outlook without issue, they are also able to cancel them on Web Outlook, so it's definitely not a permissions issue!
I created a brand new profile on the Mac Outlook, same issue, and the meeting cancellations issue only occurs for the shared calendar, not their own calendars.
An error does appear for syncing, "HTTP error. The request cannot be fulfilled."
I looked to see if it's a known issue, could not find it.
@Ash365 I have this issue as well, and it's happened to multiple people in my org. It persists after a full uninstall and reinstall of the product (though it appears that the issue subsided temporarily). My organization has observed this issue since April 9th, 2019, which leads me to believe it was present in the March 2019 Release. Another member of our organization was running version 16.13 (Build 180513) and did not experience this issue.
I am not signed up for Office Insider and my org only runs stable releases. I am currently running Version 16.27 (Build 19071500).
Please let me know if you need any other information-- thank you!