Archiving shared mailbox

Hi at all,

I'm trying to archive an shared mailbox on Grommunio server with Mailstore.

I have always_bcc set to archive all incoming and outgoing emails. But this archiving only INBOX and SENT. For full mailboxes I'm archiving additionaly via IMAP for all other imap folders.

But how to access shared mailboxes without EWS? In Mailstore I can archive multiple mailboxes with master password:

The connection ist configured as IMAP-TLS

But with this I got the following error message

Ein Fehler ist aufgetreten.
 Authentication method 'Plain' is not supported. Consider using 'Auto' or use another method from the list returned by GetSupportedAuthenticationMethods.  

The mailstore user (full user in Grommunio) is a full permission user on the shared mailbox buchhaltung@.

How do you archive shared mailboxes?

Thanks
Ulf