Hi I just use the spawn.json to set the spawn conditions for Wither Skeletons and Grue, I whant to deny all other natural spawnings but mob spawner should work. But with my current configruation mob spawner doesn't work. 

https://pastebin.com/QUexqJWF

I don't use onjoin so why mob spawners doesn't work?

Yes vanilla mob spawners. I am not sure if this could cause issues, but I use MobsPropertiesRandomness in my modpack too. Because I add custom equipment to wither skeletons.

What kind of mob spawner is it? The vanilla ones?

Seems you solved it :-)

Sorry, have been somewhat busy. Let me read up

any help? Cursed Earth from ExtraUtilities is still working, but spawners are broken with my config. Wither Skeleton spawners are working on the defined y levels but not above, why? It would make sense if I've set onjoin, but this isn't set in my config!

Solved, this configuration now works:

[
  {
    "dimension": 63,
    "mob": ["minecraft:wither_skeleton"],
    "result": "allow",
    "minheight": 10,
    "maxheight": 30,
    "healthmultiply": 0.5,
    "damagemultiply": 0.5,
    "minlight": 0,
    "maxlight": 1
  },
  {
    "dimension": 63,
    "mob": ["grue:entitygrue"],
    "result": "allow",
    "minheight": 40,
    "maxheight": 90,
    "healthmultiply": 0.1,
    "damagemultiply": 0.1,
    "minlight": 0,
    "maxlight": 1
  },
  {
    "dimension": 63,
    "spawner": true,
    "result": "allow"
  },
  {
    "dimension": 63,
    "spawner": false,
    "result": "deny"
  }
]