I added the mod to our modpack for testing.
Without Custom Mob Spawner (CustomMobSpawner 3.10.1) it works
within it crashed

crash-2017-06-06_13.23.46-client.txt

i don´t know why?

@sofifreak Use www.pastebin.com to paste your crashreports. Nobody wants to download something, if they aren't sure what it is they're downloading. (Viruses, Trojans, etc).

The crash says the problem is Custom Mob Spawner:

java.lang.NullPointerException: Ticking entity at drzhark.customspawner.EventHooks.onLivingDespawn(EventHooks.java:142)

The problem Entity is your Pig named "Schwein":

-- Entity being ticked -- Details: Entity Type: Pig (net.minecraft.entity.passive.EntityPig) Entity ID: 1231 Entity Name: Schwein

There's nothing about this crash report at first glance that makes it look like Stellar Sky is at fault.

How often does this exact crash occur?

The crash happened every time I logged in.
When i remove the Mo Creature and Mobspawner Mod it works. And differently when i remove the Stellar Sky mod it works too.

I thought I just ask on something such is known.
I opend an issues at the Mob Spawner Mod too

I found the Stellar Sky mod at cruseforge and he is awesome.

Can't diagnose this issue from here(Stellar Sky).
Please contact author(s) of Custom Mob Spawner.

No response for weeks, closing this now.