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Display MoreAsk them to share their client log:
1. open your minecraft installation path
(enter %appdata%/.minecraft in the file explorer )
2. open the logs folder
3. open the "latest" or "latest.log" file with a text editor
4. copy and paste the content of the file to https://mclo.gs/
5. send the link here
Thank you! We read the logs and found the problem. Apparently, it was the TLauncher.
Reinstallation helped, so everything is fine now 
Both my friend (the initial owner) and I have the right to launch the server.
They get following error: Internal Exception: io.netty.handler.codec.DecoderException: Java.lang.IllegalArgumentException: Unknown recipe serializer adorn:brewing every time they try to enter server.
I don't get any errors, everything works just fine. Their mods folder contain exact same files as mine (copied), so all mod versions are correct.
The logs: https://mclo.gs/dY5ztAb
Any help/explanation welcomed!
