I had this problem when the last but one patch dropped. BepinEx needed updating, which already had been updated and uploaded soon after the patch dropped. In the first instance I always move my folder with all the mods and restart Valheim: if it still doesn't work is the BepinEx, if it does work then I move the mods back one by one until I find the one that breaks it, and then just go look for an update for that mod.