'Cannot Expand folder' on shared mailbox in cloud


I run EX2016 CU21 on-prem in full hybrid mode with Exchange Online (EXO). I have several users who's personal mailboxes are still on-prem but who have been granted full access permission to the shared mailboxes that were migrated to O365 via a mail enabled security group that's also syncd to AzureAD. The problem we are having is intermittently when they open outlook and click the shared mailbox they get a message indicating 'cannot expand folder' .

What might be causing this?

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