Pro-tip: very occasionally a mod updates its code to fix a bug, but keeps the same filename. In this case, your game will not download the new version of the mod when playing multiplayer because it thinks it already has the correct version. Therefore, in multiplayer if you encounter bugs or crashes which might be caused by a mod, it is worth trying this: quit the server, delete the server's mods sub-folder and re-join the server to let it download all the mods afresh.