So, the message "Login failed, please check your username and password." shows up whenever I try to join/create any multiplayer room, I've done everything I could, changed accounts, uninstalled and re-installed the game, and also switched between Steam and website versions in an attempt to fix this error, I've also changed my password multiple times. Still, I never managed to fix it by myself, I've seen threads exactly about this error and tried the solutions in the replies, but none worked, I tried typing /login in the chat with my username and password, and nothing happened, I logged in this same account on another PC and this error didn't show up, I don't know what is wrong with my computer, it only happens on it, this is frustrating.

I don't see any failed login attempts for your account, is this the account you're trying to play with?
If you can log in on your other PC and play online then your stored credentials on your main PC are likely incorrect. Please make sure they're correct by going to Login screen ("Account" button on top right while in main menu, then "Switch account") and pressing "Log in" button.