Ubuntu 20.04 LTS port listening / reachability issues

I'm having issues on my Linux (Ubuntu 20.04 LTS) server and need an experienced person to assist / find the issue. The server has some services running, which are open in the UFW and reachable from the public sources (example: mail-server is reachable and works with no issues). When installing for example Squid proxy, or running a listener on new custom port (with ncat for e.g.), the port cannot be reached, even if allowed in UFW (or even if UFW is disabled). When I for example stop currently running & reachable nginx (port 80, 443), and run squid on port 80, it works .. So, it seems that all NEW services which are listening on "new" ports, are not reachable. For unknown reasons..