For OBM v5.0.1.4 or before, please:
Upgrade your OBM to a newer version.
Delete the obm-delta.tmp or/and obm-chksum.tmp files from your “temporary backup directory”, (e.g. C:\Backup\MSExchange\1137065709669).

For later versions of OBM, you can simply do step 2 to remove the problematic files which were left behind during the backup operations of previous versions.