[FIXED] Cannot connect, then cannot exit game

[Tsort142]

[EDIT] **If you encounter the same kind of problems, I finally found out what was wrong : it was the Killer Network Manager software that was causing all this. I de-actviated it and it fixed all the issues. **

I'm having trouble connecting to the game. It only works if it's the first game I launch right after starting up my computer, but then again, after finishing a game, I get stuck on the ending screen. Also, everytime I run the game, no matter if the connection failure happened right after a game if I was lucky enough to get in or on starting the game up, I cannot exit the game, the exit "YES" button brings me back to the main menu.

Additional info : I'm having the same problem with the game Card Hunter. It will only connect to the server if it's the first game I run right after turning my computer on. I also had trouble connecting to Marvel Heroes 2016.

This is not a new computer, everything used to run fine. Also, my gf's computer, running on the same modem, work fine. She can use Magic Duels without any bug. So there's something in my computer that's wrong. I've tried many things : restarting, restarting modem, reinstalling games + steam, opening firewall ports, deactivating anti-virus and firewall.... to no avail.

If anybody can help or has an idea about what's going on, it would be appreciated.