Can't find/set network drive as path for users

tried the search but since you can't select the Serv-U-forum as source there are too many results and I couldn't find a solution

Installed Serv-U trial on a server 2012 machine, gave the Serv-U service the Administrator user to grant access to the network.

A NAS is mapped as K: and can be accessed by \\nas-name with previously saved user/password on the Windows-Server machine.

When I create a new user in Serv-U and try to select a folder on the NAS as the user's home path, I only see my local drives. If I manually insert the \\xyz path it seems to be tried to get access for 2-3 seconds but then goes back to the screen with my local drives.

So my Serv-U installation is not able to browse/find/be forced to open network share.