Record the failure before changing settings
Take a screenshot or copy the error text. Note who hosted, who could join, and whether the failure happened before the lobby, while loading a run, or during combat. A disconnect is not automatically a version mismatch. This checklist helps separate possible causes; it does not claim one fix covers every connection error.
If one friend can join and another cannot, compare the affected player's setup with the working one. If nobody can join, a fresh test lobby may be more informative than asking every player to reinstall. Keep an ongoing run out of the experiment. A disposable session lets you test without making that run the cost of each failed attempt.
Compare the build each player is actually running
Ask everyone to report both the selected Steam branch and the version displayed by the game. "Mine is updated" leaves an important gap: two installations can be updated on different branches. If the group decides to align branches, let the downloads finish and relaunch before trying again.
For branch-selection details, use the main versus beta guide. Do not choose a branch solely because a forum reply mentions a newer number. The useful question is whether the whole group can reproduce a working session on the same intended setup.
Reduce the mod set without risking a run
Write down the enabled mods and dependencies before changing them. Use the mod authors' instructions to disable optional additions, then test a new session. Do not load a valued modded run after removing the content it uses. If you cannot establish a clean test safely, stop and check that mod's support instructions first.
If the clean session works, restore additions in a recorded order, one small set at a time. When a test fails, you have a much smaller group of changes to inspect. That narrows the investigation; it does not prove that the last mod is defective. A missing dependency, an incompatible combination, or different settings may need attention.
The Workshop and save guide covers the wider setup. This page's job is narrower: find which change separates a working lobby from a failed one.
Verify files when the installation itself looks suspect
For crashes or missing game content, Steam provides an integrity check under the game's Properties, then Installed Files. Close the game, run that check, and wait for it to finish before launching another test. Valve notes that some local configuration files may fail verification without indicating a damaged installation.
Verification does not establish that your Workshop set works together. Keep the game-file result and the mod result separate in your notes. Do not erase a save folder or disable security software to work around an unexplained join error. Those steps add risk without showing what caused this failure.
If the matching clean setup still fails
Collect a short report with the host and guest versions, operating systems, exact error, whether mods were disabled, and the point where joining stopped. Describe the shortest sequence that reproduces it. Before sharing logs, remove personal paths, account details, or other information that is irrelevant to the fault, and use the developer's official support or feedback channel.
Avoid claiming a server outage from a single failed lobby. Avoid calling it fixed because one attempt succeeded. Retest the same arrangement once, record it as your working baseline, and then return to the multiplayer strategy guide or co-op builds. Leave further setup changes for after the session.