Hello Alexander,
This is a strange problem and has to be analyzed at OS level.
It would be helpful if you can provide more information here:
What is version of MaxDB ?
Did you try running the backup again with OVERWRITE = YES (but be carefull, copy or move the last successfull backup to another location before you try this).
Are you SAP Customer ?? Do you already have a CSS message open with us regarding this issue ?
Let me know.
Regards, Yashwanth