My friend has a laptop that he runs a server on for just me and him to play on. This server always worked, whether it be modded or vanilla, there were no problems at all. One day it just stopped working, we changed absolutely nothing between the two days but it just stopped working. We've checked the internet for hours but no one else seems to have the problem we have to this extent. When he hosts it I can ping it, see the motd, the amount of players, but when I try to join I get an "IOException: connection forcibly closed my remote host.". I tried to run the server myself and I can join it fine but he gets the exact same error. I decided to try to connect to his server from a different connection with the same computer and it worked perfectly fine, but this connection won't work. We've tried:
A VPN and that worked fine, but was unplayably laggy.
Checking absolutely everything we can, all firewalls are off.
It's port-forwarded correctly (because I could join on another network).
There're no banned IPs (Because when I hosted it was a fresh new server).
There're no plugins that are harmful to the server, no plug-ins at all for that matter.
We've factory reset our routers.
We've tried everything and nothing will work at all. Please help