Generic server connection issue on external user joining

I dunno if this belongs here, but me and my friend are unable to connect and play mod-less on a fresh new server (hosted by me)

As soon as the server starts on my side everything works fine, but as soon as my friend joins the server, the server process terminates. i get disconnected and we get a very generic error with no clue on what caused it.

[Error] Dedicated server process terminated prematurely.

We both have both client and server mods all disabled, is this still an issue related to mods even tho the logs don't indicate an issue with them?

I had trouble finding help online so i'm making this post.

The attached logs are my client and server log, plus my friend's client log (client_log_2.txt), which after different attempts at connecting shows:

[Connect] Got join server cancel from lua.

The issue might be related to hosting a server on linux in some way (maybe some missing permission, or something in the connection protocol) ?

Sorry for the extremely vague report on my part, but the lack of information about this issue is what is making me write this post in the first place, sharing the logs is the most i can do regarding accurate information, as helpful as it gets.

attached files:

1- my client log

2- my server log

3- my friend's client log

client_log.txt master_server_log.txt client_log_2.txt