This problem occurs in some sessions, but the same session can work successfully and the next time can work unsuccessfully, this is the strange problem.
I already isolated problem, infact this occurs only on one HP-UX of three servers. It has 8.13 version but problem occurred before to update from 7.0.3 to 8.13.
I updated DP to 8.12 few days ago so my unique 8.13 client is this server where often backup fails, the others client have always 7.0.3 version. But this problem happened before update.
I opened case but HP thinks to be hardware problem! Mmmmmmm