the source of this issue comes from having an older version of hydrocraft subscribed, even if it isnt activated. Basically, PZ loads mods based on the mod id, normally all mods should have a unique mod id so this isnt a problem, but previous hydrocraft mods and this one have kept the same mod id for new mod pages to make it easy to update to the new mod when the maintainer of the mod changes without needing to manually edit save files to load a different mod as if it isnt fixed, it will delete all the items from the save, ruining that run.

But because of this, having the old mod subscribed causes some issues because even if the old mod isnt activated in PZ. The logic for mod loading is flawed, and it still tries to load files from the old mod. So 2 possibilities of the issue. Either your server loading the old mod as well, and complains the players are missing files as the players only have the newer mod. or the opposite, players are trying to load in with both mods which the server only expects the newer files. So make sure that the server is not loading both and neither are the players. Its worth manually checking the mod folders as well, as steam isnt very good with regards to deleting mod files and sometimes leaves stuff around even though youve unsubscribed.