Hello Michael,
It seems that you are running out of disk space on your log volume.
When the Log Area is FULL, the backup activity is cancelled, and the AUTOSAVE LOG is deactivated. I asssume this is what is happening in your case.
You must always ensure that you explicitly reactivate the AUTOSAVE LOG when you have space in the file system again. If you do not activate AUTOSAVE LOG, the log area may become full again.
If this is not the case and you have enough space on your log volume and still face problems with backups, it would be helpful to check the KnlMsg file for further analysis. Please attach the KnlMsg to the thread.
Refer to SAP Note: 1377148 - FAQ: SAP MaxDB backup/recovery
Regards,
Yashwanth