Player disconnecting immediately after connect

Hello!
I have a strange network problem. I’ve been playtesting my game with 4 players for a while now. There wasn’t any problem for a long time, but last time we wanted to play, and everyone could connect just fine except one player.
I have a DB which tracks every open servers external IP address and port.
I’m the listen server, router portforwarded. Everyone from different networks can connect no problem.
But the one PC from the same network have a problem. It connects fine, pawn created, possessed, but then it just stops, and after 1 minute, it times out.
I checked my whole code, debugged, but found nothing. Then had an idea and downloaded MutliplayerShootout project, built, and tried out the same (instead of the lobby browser it has, i used the "open " command line), and it had the same problem. Connecting in, characters created, then it stops, and disconnect after a while.

In my project I made a workaround where I store in the database the servers public and private ip address. On the client, it checks if the server public ip is the same as the clients, if so, it uses the private ip instead. Not an elegant solution, but works for now (probably wouldnt work in all the cases).

I’ve tried out with a project of mine that was done years ago (dont know the exact engine version), and it had the same problem. Probably its with my network, but not sure how could i resolve it. Any ideas?

Okay I managed to solve the problem.
I’m using UPC Connect box router, it has firewall inside. When I disabled “Port scan prevention” in the firewall, everything started working.

Just leaving this here in case anyone have the same problem