Our modded co-op campaign broke after an update and nobody remembers what we installed

Four of us have been playing the same modded cooperative campaign since January. It began with three harmless additions: a better map, larger storage, and a cosmetic pack. Then someone wanted new enemies. Someone else found a weather overhaul. By spring, our “small quality-of-life setup” had enough moving parts to require a loading screen just for the loading screen.

The game updated this week. Now the host can open the world, but the rest of us disconnect while joining. If he creates a new unmodified world, everyone connects. That suggests the problem lives somewhere in our setup, but “our setup” is currently a collection of subscriptions, manually downloaded files, and one folder named OLD DO NOT DELETE.

Nobody acted as the librarian. Each person installed what the group chat linked at the time. Two links are dead. One mod changed names. A person who stopped playing months ago may have configured the original load order. We have screenshots, but they were taken after different sessions and do not match.

The host wants to remove half the mods until the world opens. Another player is worried that loading the save without a content mod could permanently remove items or structures. I suggested working from a copied save, but we are not even sure whether every relevant file is inside the visible save folder. We have reached the point where caution and confusion look identical.

Starting over would solve the technical problem and kill the campaign. We are adults with inconsistent schedules; rebuilding four months of progress is not a charming fresh start. At the same time, spending three evenings repairing a world so we can relax together has a certain tragic symmetry.

What is the safest diagnostic order for a mess like this? My proposed plan is to freeze the current files, document exact versions, make multiple copies of the world, confirm the base game and network work, then add the known required mods in controlled groups. Is there a better way to identify which additions are save-critical before testing? More importantly, what record should we keep afterward so the next update is a maintenance task instead of archaeology?