I'm trying to set up a zoo server for a couple of friends on Aternos, and I really want to have this mod in it, but whenever I add it, no matter the version of the mod, it won't let me join the server, even with the same version on both the server and in my files, idk why this is happening but it's only this mod.

Tells me "Failed to synchronize registry data from server, closing connection." every time I try to join, the Aternos log doesn't tell me why, but I just looked at the MultiMC log and it looks more helpful, looks like something with spawn eggs.

[15:06:13] [Netty Client IO #0/ERROR] [ne.mi.fm.ne.NetworkRegistry/NETREGISTRY]: Channels [environmental:login,untamedwilds:main,alexsmobs:main_channel,citadel:main_channel,environmental:play,abnormals_core:net,geckolib3:main] rejected vanilla connections
[15:06:15] [Render thread/ERROR] [ne.mi.re.GameData/REGISTRIES]: Unidentified mapping from registry minecraft:item
untamedwilds:bear_spawn_egg: 2021
untamedwilds:big_cat_spawn_egg: 2022
untamedwilds:shark_spawn_egg: 2032

[15:06:15] [Render thread/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Missing registry data for network connection:
minecraft:item: untamedwilds:bear_spawn_egg
minecraft:item: untamedwilds:big_cat_spawn_egg
minecraft:item: untamedwilds:shark_spawn_egg

[15:06:15] [Netty Client IO #1/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Failed to load registry, closing connection.
[15:06:15] [Netty Client IO #1/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Connection closed, not continuing handshake
[15:09:03] [Render thread/INFO] [minecraft/ConnectingScreen]: Connecting to 1hourzoo.aternos.me, 52333
[15:09:06] [Render thread/ERROR] [ne.mi.re.GameData/REGISTRIES]: Unidentified mapping from registry minecraft:item
untamedwilds:bear_spawn_egg: 2021
untamedwilds:big_cat_spawn_egg: 2022
untamedwilds:shark_spawn_egg: 2032

[15:09:06] [Render thread/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Missing registry data for network connection:
minecraft:item: untamedwilds:bear_spawn_egg
minecraft:item: untamedwilds:big_cat_spawn_egg
minecraft:item: untamedwilds:shark_spawn_egg

[15:09:06] [Netty Client IO #2/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Failed to load registry, closing connection.
[15:09:06] [Netty Client IO #2/ERROR] [ne.mi.fm.ne.FMLHandshakeHandler/FMLHANDSHAKE]: Connection closed, not continuing handshake

Can't help you there, there's absolutely nothing special that could single out Bear, Big Cat and Shark spawn eggs. If there was any issue with them, it should also trigger for every other spawn egg. My only guess is that you have added/removed mods on the server without regenerating the world

I looked a bit into the problem, and found a thread on the Forge forums on a similar issue

Update. Fascinating. I passed in "--forceUpgrade" when launching the server and this seems to have resolved my problem. I can now connect the client to the server. Is this something forge or minecraft handled automatically previously? I've never had to do something like that before.

From this thread

Although I've installed mods without regenerating the world before, I've regenerated the world now, and the same issue shows up again.