2 shared Mailboxes with same rights, cant send mail.

Hi. 

ich have exchange 2016 onPrem with 2 Mailboxes 

Mailbox A is a created as shared Mailbox years ago 

Mailbox B is via powershell migrated from regular Mailbox to SharedMailbox  with powershell these days

set-mailbox -identidy MAilboxA -type shared

checked in ECP 

On Both Mailbox i have the Full Access Right and no send as right. 

They look identically 

When i create a new Mail in Outlook Client M365 actually patched. 

i change the sender field to the Mailbox the mails should come from, 

Autocomplete is cleared, address is selected from Addressbook. 

from Mailbox A this is sucessfull 

from Mailbox B i after click the send button, i get a error that i have not the right to send mails on behalf the mailbox.

Any Idea 

i cleared autocomplete, did a new Outlook profile.

On ECP the Mailbox rights are looking identically.