Indeed it doesn't identify the un-reachable host, but we do get some more information, we get the bpdbm pid. Now that pid may not be helpful right now if there are no bpdbm logs, at the same time it suggests if we configure the bpdbm logging directory we should be able to trace the log messages associated with the pid.
Go ahead and create the bpdbm logging directory on the master server, netbackup/logs/bpdbm, once created increase the logging level for bpdbm via the NetBackup administration console -> master server host properties -> logging, or by configuring the 'BPDBM_VERBOSE = 5' within the master's bp.conf file. Once the verbose levels have been changed, bounce the services on the master or wait for a good time to bounce the services.
Wait a bit to get another image cleanup job to run, or kick one off by running I believe, bpimage -allclients -cleanup. Let the job fail, it should fail the same way, then go ahead take note of the new bpdbm PID. Now go to the netbackup/logs/bpdbm/ directory on the master server, and then grep out the pid from the log file, e.g. 'grep <pid> log.date'.
Look in the grep output for '<16>' those are usually severe messages, also try looking for the 'host unreachable', or 'status 47', or simply '47'.
Anything valuable in there?