If you recently upgraded your Yogscast Complete modpack to version 18.104.22.168-RR-YOGS the players re-joining the server may find that their client is crashing, or worse, the server is crashing because of data held in their player.dat file.
The cause is a change in the Potion IDs that have been set for the Ars Magica 2 mod. Players who logged out before the update with a status effect applied to their character will have invalid data in their player.dat file. At worst this will cause a crash, at best the status effect may change to a different status effect.
The easiest fix is to simply delete the player.dat file but you can remove just the status effects using the NBTExplorer tool:
- Run NBTExplorer
- Choose File>>Open or click the 'Open' icon
- Choose the player.dat file (it's located on the server at /world/players/<player-name.dat>)
- Find the 'Active Effects' node and open it (Double-click or click the expand icon)
- Inside will be the data for each active status effect, click on one to highlight it and press the delete key (or click on the delete icon)
- Once all the Active Effects have been removed, choose File>Save or click the 'Save' icon