Admin Privileges – If you are not running Steam as an administrator, you may face the Risk of Rain 2 not working issue.
Blocked Game Ports – When the router you use has assigned you the same port that the game uses for some other purpose, then you will encounter the said problem.
Corrupt Local Files – The corrupt game files and data might cause this problem.
Firewall issues – If your Windows Defender Firewall or third-party antivirus is blocking Risk of Rain 2, then you might not access few features in it.
Many reasons cause Risk of Rain 2 multiplayer not starting problem, such as: How to Fix Risk of Rain 2 Multiplayer Not Working Issue