"cannot find the calendar folder for this item" - calendar items stuck in outbox


New Member
Outlook version
Outlook 2016 32 bit
Email Account
Office 365 Exchange

We have an Office365 subscription, I believe i'm running Office 2016...

Have an unusual glitch on just one of our PC's.
When forwarding a Calendar Item to others, it sits in the outbox with no "Sent" time... (an indication that it's not going to send).
When trying to open the item from the Outbox, the error message "cannot find the calendar folder for this item" is displayed.
No other calendar issues, and I'm not sure why I only have this issue with one PC.

Any suggestions?

Thanks! :)