we migrated all our mailboxes from on-prem to exchange online.
We have a few public folders, that are still left on onprem.
They are working as expected, except one thing that is not working anymore.
If a user moves a mail from one folder into another, or in a subfolder, all users with access to this PF will get the message "Cannot expand the folder. The set of folder cannot be opened. Microsoft Exchange is currently busy. If this message is still displayed in 30 minutes, contact your Exchange Administrator." After restarting the Outlook, everything is back to normal again.
Our Exchange is an EX 2019 with the newest CU and SU.