I haven’t seen these logged since the ZP-575 fix in 2015.
The event is currently in the sent items folder on the server and not being synced to Outlook. This is an old meeting request from March 2016. It used to sync with 2.2.12 but no longer with 2.3.7.
Attention needed: 1 messages need attention because they could not be synchronized Broken object: 'SyncMail' ignored on '2017-08-12 10:33' Information: Subject: 'Accepted: blah blah blah - From: '"USERKD" <USERKD@domain.tld>' Reason: (4)
Viewing the event with webaccess I see from USERKD to a single external recipient.
When: = This item is recurring
Location: = External site
“This meeting is not in the calendar, it may have been moved or deleted.”