It seems to me that a few things are the possible culprit. It appears that root on the client does not match root on the NFS server and is potentially disallowed from altering ownership of files either from a UID perspective or who owns /export on the NFS server.
The issue here has nothing to do with the useradd command itself because as you stated in the comments you cannot chown files/folders from the client on the export at all. This can be due to the way it was exported, a UID/GID mismatch, or how the ownership of the folder is set up on the server side.
As I previously mentioned you can check the permissions of the folder server-side, and look for UID/GID mismatches and make sure the ownership of the exported folder is correct. Be sure to re-export the folder and re-mount it any time you change a setting to ensure it takes effect. You may also consider setting anonuid=0,anongid=0.
One last thing is that if you are using NFSv4 try it using NFSv3 to see if the problem happens there (nfsvers=3).
A lot of OpenSuse specific troubleshooting information on this very topic is in a thread I found here on the opensuse forums, although they did not appear to resolve the issue.