Whenever a Mob Captain would spawn in, I would have to go into Creative mode just to kill it.
I have a hunch that a datapack is conflicting, not sure which one is messing with Mob Captains though.

I attached the datapacks I use on my server, maybe this will help narrow down the problem.
For some additional information, I'm playing on a Minecraft 1.20.1 hosted on Aternos.

To help debug this, can you run the following command while standing next to a captain and send what it outputs in chat?

This will get all their data, including the Invulnerable flag and the Tags array, those should give more information

data get entity @e[tag=ps-mob.captain,limit=1,sort=nearest]

I assume I just type "/data get entity @e[tag=ps-mob.captain,limit=1,sort=nearest]" in the chat, correct?

This is the server logs:

Yes, you ran the command correctly

Hm okay, looks like your server is modded, making things a little harder

Since it’s complaining about the message being too long, can you try these 2 commands one by one instead?

data get entity @e[tag=ps-mob.captain,limit=1,sort=nearest] Invulnerable
tag @e[tag=ps-mob.captain,limit=1,sort=nearest] list

edit: corrected the commands

In case it helps, these are the mods used in the server.

data get entity @e[type=ps-mob.captain,limit=1,sort=nearest] Invulnerable

tag @e[type=ps-mob.captain,limit=1,sort=nearest] list

The commands didn't work. I assume it's something that I did, not sure what though as the command is red (invalid) before I executed it.

Oh, my bad, it should be tag= and not type=, I typed them out quickly on phone 🙈

This came up in the chat after entering those 2 commands.

Nothing came up in the console, though.

Thats better - though it says "migongaming" as the name of the entity 🤔 I would expect it to be "Zombie" or what ever mob it is, can you add me on discord? Writing there should be quicker than on here

https://discord.com/users/346300510581817354
or add puckisilver on discord

If not you could go next to a zombie captain and run the commands like this:

data get entity @e[type=zombie,tag=ps-mob.captain,limit=1,sort=nearest] Invulnerable
tag @e[type=zombie,tag=ps-mob.captain,limit=1,sort=nearest] list

added

The problem was the No-Lag data pack. The solution was to remove that data pack - all captains spawned after behave properly.