When I play in singleplayer everything is fine. When I make the game open to lan and another player joins they immediatelly get disconnected. There are no errors on the server, there is only a warning on the client that tried to connect:
Client disconnected with reason: Internal Exception: net.minecraft.server.v1_14_R1.ResourceKeyInvalidException: Non [a-z0-9/._-] character in path of location: minecraft:\u0001\u0004\u0001\u0001

Minecraft 1.20.4 Forge 49.1.0 PrimalMagick 4.2.0-beta

I scoured through the files with regexp but didn't mange to find any file that has a non [a-z0-9/._-] character in it nor empty value after "minecraft:". Same with jsons: all the references seem to have a correct format.

The issue is not 100% reproductible. I managed to connect three times (out of dozens) when I was trying to figure out exactly what helped or what really causes the issue.