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.

Despite my best efforts, I've been unable to reproduce this issue. If it occurs for you again and you can acquire a log file or crash/disconnect report that I can examine in more detail, please open a new ticket. In the meantime, I'm going to close this.