Fix a Black Screen When Joining Multiplayer

If the screen turns black only after joining a friend, have everyone leave the lobby, fully restart both Steam and the game, confirm the same game build, then let the host create a fresh private lobby and send a new Steam invitation. ^[01]

Isolate the cause in this order

• Join the new lobby one player at a time so you can identify a specific client.
• Verify game files on every machine and install any pending update.
• Disable BepInEx and all mods for the test. Mixed mod versions or settings can break joining.
• Allow Steam and the game through the firewall without permanently disabling security software.
• Test a different host. Do not reuse an old invitation or stale lobby.

If the game is black before any lobby is involved, use startup and display troubleshooting instead; that is a different fault. ^[01]

Report a reproducible case

Record the game build, which player hosted, the exact join method, whether sound or movement continues behind the black screen, and whether a clean unmodded lobby works. The available direct material is community troubleshooting rather than an official root-cause statement, so these steps are Reported and should be retested after patches. ^{[01] [02]}