17 Dec
                
                    2015
                
            
            
                17 Dec
                
                '15
                
            
            
            
        
    
                5:52 p.m.
            
        This looks like problems with the file system permissions.
Addition to my previous error with the global file being a directory:
Seems like Steffen was right on the spot here anyway:
The directory for the mailbox was indeed owned by me with all 775 permissions and also the MailingLIsts/mailboxes.
But the top level Mailinglists had for some reason a 555 permission and "doveadm delete" tried to write a temp file into this for some obscure reason and failed which failed the delete.
Found it by strace -f doveadm delete
Now fully solved!