how to handle this error?

NBU 7.7.3

so it has become a policy to immediately delete backup disk images of successful jobs that are part of an unsuccesful job set. i use the below commands:

nbstlutil cancel -backupid <id number>
bpexpdate -backupid <id number>

sometimes the commands would complete successfully, sometimes i get this error:

[root@master admincmd]# nbstlutil cancel -backupid server01_1491276905
Operation not successful: status = 220 (database system error)

what does this error indicate?

regards,

Rino