thanks for your reply. Yesterday I changed the ndmp port to 9000 at my backup server (C:\windows\system32\drivers\etc\services) and it doens't work. I left the ndmp port at 9000 - maybe the solution works also without changing the port setting.
Today I talked with a technician and we got it. 🙂
You get the issue if you try to make the choice through the "Active Directory Domain"-view. If you make your choice through "Userdefinied choices" (first you have to define the share, then you are able to choose it), you can proceed your backups and restores as normal. The only thing is that it takes some seconds till the tree is open.