I made a LAN server with my friends with some mods on forge 1.16.3. We were playing for 3 weeks and it was ok, but after we tried to upgrade to a new version we got this problem. Even after opening the save before an upgrade we still had this error.
mods list:
astikor carts, biomes o plenty, voxelmap, guard villagers, jei and simplefarming.
We also tried to download the same mods and forge version but it still didn't work.