I couldn't find out the reason for the errors. But I found out, that the errors are probably depending on the win-clients.
I made on two different clients (one XP, SP2 the other NT, SP6) new shares. And, what shall I say, it works. On the SME I deleted all old backup-jobs. I installed rar-3.1.0-1dmc.i386.rpm and smeserver-backup2ws-0.0.1-20.noarch.rpm.
Maybe it helps other users running in the same problem.