Thank you for your answers!
The share is on my Server HD in the folder "/Shared Items/Share Points/TheFolderIWantToShare" (could this be a problem?).
The user has access to the service (Edit Access To Services; File Sharing is checked) and he has read and write permissions to the folder.
When I enter "ls -ald /Volumes/Server HD" to the terminal I get this output: "lrwxr-xr-x 1 root admin 1 Jan 28 16:58 /Volumes/Server HD -> /"
The File Sharing service has already worked for the user. But one day, when I tried to connect to the server, I always get the following error message:
Title: "There was a problem connecting to the server “myserver”."
Message: "The share does not exist on the server. Please check the share name, and then try again."
The client is in the same network as the server. When I give the user admin privileges, he's able to mount the folder :S
I solved the problem myself.
I have moved the Shared Folder to my second HDD in my Mac Mini.
I think there were some permission difficulties on my System HDD.
The Apple Care Technical Support also helped me with the Problem.
Thank you again for the help.
Special thanks to the Apple Enterprise Technical Support. You helped me a lot with the problem !!