Hello Chris,
same problem here a long time ago. Probably there is some connections to MaxDB which cause your issue. In my particular case I was connected via dbmcli utility to the database before the restore, but the remote session went down for network timeout leaving a process in hang status on the OS level.
When later on I started the restore I had the same error. I manually kill that process and the restore started and finished successfully
Regards,
Valerio