'Cannot Expand folder' on shared mailbox in cloud

[shockotechcom]

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?

[#2]

@shockotechcom 

-------please help urgent -----

Hello,

I have added the additional mailboxes under the account setting
and when i open this mailbox i am getting error msg "cannot expand the folder"
I have full access to user mailbox from the owner of this mailbox

I have tried removing and adding and restarting but nothing helped to resolve this issue

please help