FAILURE DUMP SUMMARY:
nd02 Nvedata lev 1 FAILED [too many dumper retry: [request failed: timeout waiting for REP]]
nd02 Nvedata lev 1 FAILED [data timeout]
nd02 Nvedata lev 1 partial taper: successfully taped a partial dump
nd02 Nvedata lev 1 FLUSH Got empty header

what is the cause?

a Full backup works just fine but incremental backup has always fail with that error.