The messaging Interfaces have returned an unknown error.

Keep getting the same error for different shared mailbox for multiple users 

Environment - Exchnage Hybrid 

users are migrated to cloud - all cloud now (remote mailboxes on premise)

shared mailboxes/calendars - hybrid  (rmeoe mailboxes on-premises) 

Issue is not happening in OWA, only Outlook Desktop 

Office suite is updated to latest version 

Profiel reconfigure has been done - not helped

permissions rmeoved for delegates and calendar , but issue still persists

Error happens only when modifying an existing event in Calendar of a Shared Mailbox by a delegate 

Need suggestions from whoever has seen this before and from Outlook MVP's / Product Group Team.