Problem to contact other servers in the same net

I have set my UCS properties for the firewall in the following way:

ucr set security/packetfilter/disabled=true
ucr set security/packetfilter/defaultpolicy=ACCEPT

But when I try to do an NFS mount or connect to a MySQL database on another server, I get no connection.
The NFS mount and the MySQL connection work fine on other clients.

How can I find out what fails?

I think I found the problem.

As univention only accepts a real domain name, I entered the one set up for my ISP router, but within my LAN I have only three letters as domain.

DNS gives me now the IP of my ISP router instead of the server I wanted to contact.

What was misleading me, was that I could ping the server, but I did not realize, that it was the wrong IP