I have few clients with 4.5MP6 and VSP enabled. At some point NBU client
is trying to make snapshots for volumes and it could take more then
30-45 mins in total. After that client is unable to communicate with
server (TCP connection timeout in bpbkar file), 
server thinks backup is still in progress, keeps tapes mounted but
unable to communicate with client. The bad thing - I can't cancel the
backup using *job* commands. May be it is good idea to open the case but
I'd like to ask the group first - may be there is a way to tune timeout
somewhere to resolve the problem.

BTW, disabling VSP and OTM resolves the problem...

Dmitri