There is currently an open issue with Shared Mailboxes from Microsoft. This article describes what is affected and how to get around it.
Affected Clients: Outlook Web, Outlook (Preview) Client for Microsoft
In the new Outlook Client for Microsoft and in Outlook Web, Microsoft is currently facing issues which also affect Mailissa. Please note that there is a difference between the classic Outlook Client for Windows (right) an the new one (left).
While this issue is open, Mailissa will stop working after the authorization process in Shared Mailboxes and appear as a white screen.
As a workaround, please use the classic Outlook Client for Windows in the meantime. We will inform you once this is fixed. If you have any questions, please contact us via Mail or open a new ticket.