I am getting this while running a server, it is spamming the console.
[22.12 18:50:29] [Server] [Server thread/WARN] [de.te.va.en.ModEntityEventHandler/]: Unknown zombie entity type TranslatableComponent{key='entity.dungeons_mobs.armored_zombie', args=[], siblings=[], style=Style{ color=null, bold=null, italic=null, underlined=null, strikethrough=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null, font=minecraft:default}} for zombie target task

Armored zombies no longer exist, issue should be gone

Have no idea which mod it is but it's their fault for spamming the log like that, not mine.

That is not a mod, it is the server attempting to spawn an Armored Zombie and rejecting it. It is the fault of this mod.

Then ModEntityEventHandler is the server handler for entity spawns and it is the one spamming the log with that. I’d check with your server provider as to why it’s the case, I can assure you there is nothing wrong with the Armored Zombie on my end.

This is an error that occurs on any multiplayer minecraft. I've tried it in both a localhost (on my own computer) and on a hosting service (Bisect) and it occurred on both the same. While the zombie spawns well in Single Player, it appears to only do this in multiplayer when attempting to spawn. I can assure you it is not because of who I host with.

I have this issue as well:
MC 1.16.4
Forge 35.1.29
Dungeons Mob 1.0.3

[09Jan2021 18:47:41.812] [Server thread/WARN] [de.teamlapen.vampirism.entity.ModEntityEventHandler/]: Unknown zombie entity type TranslatableComponent{key='entity.dungeons_mobs.armored_zombie', args=[], siblings=[], style=Style{ color=null, bold=null, italic=null, underlined=null, strikethrough=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null, font=minecraft:default}} for zombie target task

debug log: https://gist.github.com/Yourname942/e7fdb4134d738f34b6abc82fbf04e54e
latest log: https://gist.github.com/Yourname942/de0e37675fee5323bef4c8d8d285aac6