Exchange calendar syc with Teams produce a lot of timeouts

Helmut Sz 1 Reputation point

Exchange Hybrid - 2 Server Cluster (DAG) running Exchange 2016 CU22
Mostly on-prem mailboxes with some mailboxes in ExO.

So for about 2 months now, I am facing massive sync issues with the Teams calendar.
Aproximately 90% of the sync requests time out after 15 seconds (that seems to be the threshhold Teams is willing to wait for a calendar request answer from Exchange).
The calendar will sync eventually but only after some to a lot of timeouts. This happens for every user.
I'm trying to figure out where the bottleneck might be. As it is basically working but just not nearly reliably enough, the configuration seems to be correct in general (otherwise it wouldn't work at all I guess).
Also when using the Microsoft Remote Connectivity Analyzer - Teams Calendar Tab check, all checks succeed except for:
Sending user properties settings request to the Teams middle-tier service.
"The Teams middle-tier service endpoint did not return a valid response. > The request to the user properties settings service timed out. Please retry. Timeout: 15 seconds"
Also the Exchange Servers themselves don't seem to be overly busy when checking the ressource monitor.
The Outlook calendar sync don't have any problem.

I already tried a lot of things and would realy appreciate any suggetions that can point me in the right direction.

Thank you for your suggestions

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