Couldn't create server on port 28015

My friend has a problem creating his private rust server. It’s the same error popping up over and over again. He tried to use different ports, disabled/enabled firewall, changed permissions for the rustdedicated.exe, still no success.
He can create local server, in this case others won’t be able to join. Few days ago ordered static IP.
Is there something else we could try to fix this error?
Thanks in advance.