Actually iam building a large modpack and playtested it with a friend for some time now on a rented server. Sometimes it crashed when we entered a new dimension but just because the server was overloaded. Wasnt the strongest server. One day my friend was playing and the server crashed. I wasnt avaiable and when i later restarted it he wasnt able to connect anymore. He always get following error "Fatally missing registry entries".
We already tried the following things so solve the problem:
Reinstalling his minecraft, took my mods+configs folder to his pc so we have the same files, reinstalled the server, deleted his playerdata from the map, tried a new map but its always the same. Starting a singleplayer game works just fine.