It might be my ineptitude at JSON programming, but when I tried altering the spawn file, the game I run fails to save and quit.

Furthermore, the internal server basically shuts down, and when creating a new world, it would go for some time before indefinitely freezing (needing to shut off the game via Task Manager).

Here's the entire contents of spawn.json

[
{
"mob": "minecraft:blaze",
"block": "minecraft:netherrack",
"maxcount": {
"amount": 20
},
"result": "allow"
},
{
"mob": "minecraft:blaze",
"result": "deny"
},
{
"mob": "minecraft:creeper",
"maxcount": {
"amount": 3
},
"result": "deny"
},
{
"mob": "minecraft:zombie",
"maxcount": {
"amount": 40
},
"result": "deny"
}

]

Here's the potentialspawn.json file, the only other file I've touched

[
{
"dimension": 0,
"mobs": [
{
"mob": "minecraft:blaze",
"weight": 5,
"groupcountmin": 1,
"groupcountmax": 2
}
]
}
]

This was fixed in the 1.16 version some time ago

I also couldn't find any debug messages in the in-game output either.

Version please

Minecraft 1.15.2

Forge 31.2.44

There was no documentation for 1.15.2, so I used the 1.14 documentation instead.

It seemed to work for the potentialspawn.json, but the freeze happened when I added to the spawn.json

The 1.15.2 versions are a bit dead right now. I recommend moving to 1.16.5. That's the version everyone is moving too

Ah crap, I was using this for a modpack, but thanks anyway.

I'm on 1.16.5 and forge version 36.1.1 and am having the same issue
in my case it only happens on pre-existing worlds, but newly generated ones do not freeze until I restart the game

the Json files are here
https://pastebin.com/PaMMmeSv

@Darkstar6423 version of In Control?

5.1.6

I'll check it out

I don't know if you found the cause of the issue or not but I tried running it on a server and managed to get a crash report from it.
https://pastebin.com/Q2pp59AX
I hope this helps