"New Outlook" No permisions to shared mailbox after enable

Hello.

last days in our organization we migrate couple of users. As always, users likes to test new things. In few cases we have seen problem, when after user click "new outlook" switch, they had errors when accessing shred mailbox - error about no permissions pop up. Question is: are there any limitation of this option (was not able to find any documentation)? What potentially caused this situation? 

Thank you in advance for every reply. 

Best Regards.