Some players have been reporting that after killing XJ, the game crashes and they aren't able to load or log on the character again. They also say that other/new characters work fine, but if you try to log on the character that had the crash, the client will crash again.

This doesn't happen to everyone, but people have suggested to make backups of your save file in case this happens to you.

C:\Program Files (x86)\Steam\userdata[YOUR STEAM ID HERE]\582010\remote

This is where your MHW save file is, you just need to copy SAVEDATA1000 and save it on your backup folder for future use in case this bug happens to you.