! NECRO POST BECAUSE NO SIMILAR TOPIC EXISTS ANYWHERE ON THE INTERNET !
The way I managed to fix it was exactly as follows
Duplicate your modpack
Disable all mods
Update all mods <----- VERY IMPORTANT THAT THEY ARE UPDATED FULLY
Enable them in clusters of 5-10 mods and launch, repeat until all mods are enabled
Go to a singleplayer word, open it to lan, enable cheats and try it out
If this doesn't work... I'm very sorry