If this were my nbu server? I'd shutdown Netbackup, shutdown pbx_exchange, make sure that all processes for nbu were down (bpps -x would show no active processes) and then I'd start vxpbx_exchanged, and then I'd run bp.start_all.
Check all processes via bpps -x (at an elevated prompt). Sometimes a clean restart of netbackup will fix the issues we wrestle with. IMHO, a clean start of netbackup is a great place to start. Also, if your appliance hasn't been rebooted in a long time (more than 90 days), I'd look at rebooting it.
Also, there are many fixes and enhancements in 3.2/8.2 that helps with things like this and your firmware may also need updating. Just some suggestions is all. :)