On premise public folders in hybrid scenario only working with Outlook cached mode disabled

[mocarysga]

Dear community, I have a very strange behavior with public a folder mailbox in a hybrid scenario and desperately call for help ;)

Here's what is happening right now. We are currently migrating our users from Exchange 2016 CU9 to Exchange online, which is working fine so far. Only some 30 users are still on premises, the rest has been successfully migrated to Office 365 and Exchange online. Our public folder mailbox (45 GB) still resides on premise. All of our migrated users can successfully connect to the public folders on premises and list all folders, however, when trying to access the files in a folder they receive an error message. For some users though this is working just fine. 

What we discovered today for our users that are not able to list files in a folder or copy new elements into a folder is, that without Outlook Cached mode enabled, these users start seeing folder elements and are also able to copy and delete items. Turning Outlook cached mode back on and restarting Outlook, the error message is back...

We are on a "Office 365 Business Premium" subscription and most Outlook versions should be up to date. When we disabled outlook cached mode and checke on the Outlook connections, we only saw one connection regarding public folders, pointing to our internal Exchange server. Turning cached mode back on, we saw two connection for public folders, one pointing to Exchange online and the other pointing to out internal Exchange Server. Screenshots are attached. Maybe this is something that might help troubleshoot this issue...

In case you need more information, I'm happy to provide. Thanks a lot in advance!

Sascha