If you've seen this and are also struggling with this problem, I found the answer here:
https://github.com/FTBTeam/FTB-Modpack-Issues/issues/364
1: You remove the files in 'serverconfig' folder (in your world folder),
2: Put the file(s) from the 'defaultconfigs' folder in the 'serverconfig' folder, which is located in your .minecraft folder.
(you should probably make a backup if that doesn't work btw)