Having the same issue on a modded server, and I don't see any other mention of this error online.

@Erik Herbest So do I, I think it is impossible to fix it, tried many things but didn't suceed. Maybe Mojang can work on that or we just need to see if someone can fix that.

Hey ! Did your problem got solved ? 

I'm starting a Minecraft Forge server on 1.14.4, with the Biome O'Plenty mod, bur as u guessed, he can't join me, and has the same error message as you "Failed to synchronize registry data ..." 

I've searched, but i can't find anything about it elsewhere ... 'Hope you can help !

I have the same problem but with a non-LAN moded server, i see it open in the selection menu, then get the "failed to synchronize registry data, closing connection" error message, then back to the server selection menu the server is pinging.. i only get the error message when i try to connect to the server for the first time after launching minecraft.

I found the solution!

All parties must install a fresh and completely new version of forge AND the mod(s) that they have installed. I noticed that my friend's launcher said Forge 28.0.75 (I think) and mine said Forge 28.0.5. I installed the newest version on both computers, relaunched, and it worked! Hope this helps, it did for me!

If this is still an issue in 1.14.4, please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) here.

Hey ! It stills a problem on my side, because me and my friend were trying to play and a Forge server, knowing that we had both the same version of Forge and of the mod (Biome O' Plenty). I think the problem might be coming from the version of Forge itself, which was said to be in Beta... Anyway, we went to do something else instead, vanilla Minecraft is not that much boring without mod i guess 🙂

Thx for the reply !

My friend is for some reason having this issue in 1.16.1 when he tried to join my modded aternos server. We have the same mods (latest updates) and the same 1.16.1 forge (most recent one) so I don't know why it's not working ;-;

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If this only occurs with mods, you'll have to get in touch with the mod creators. Unless we can prove this is an issue with vanilla Minecraft, this issue is invalid