S3 backup problems since last upgrade

S3 backups are failing since the last VM plugin update.

----- The following addresses had permanent fatal errors -----
060744c5550b46f
(reason: 550 5.1.1 <060744c5550b46f@openstream.ch>... User unknown)
(expanded from: 060744c5550b46f)

The 060744c5550b46f is constant and there is no such user for any account so I have no idea where it is getting this from.

Status: 
Closed (fixed)