I believe I know what the problem is. I played a little bit around with a new share as you suggested and connected it to different directories. Some dirs I could connect via nfs, some not. It might have something to do with the rights - we moved the server from a windows domain (NT) using ADS to a different domain using internal LDAP for authentication.

So all I need to do is to change the access rights what would be easy, if I would be able to mount the dirs via nfs...