If Biomes O'Plenty is present NetherEx lets only appear its own biomes in the nether. All BOP features are skipped.

Can also be found here: https://github.com/MineEx/NetherEx/issues/65

NetherEx uses their own custom chunk generator for the Nether with custom GenLayers, and their chunk generator overrides the vanilla Nether. Because of this, you will need to use their config files in order to add other mod's biomes to their Nether. See: https://www.reddit.com/r/NetherEx/wiki/configuration >> Biome Lists

There's a script on the wiki with BoP biomes + NetherEx biomes. Install that, and it should work fine.

There are so mich options. Is there no way that NetherEx automatically registers all biomes?

Would you mind giving me a link?

@JonasCiss https://www.reddit.com/r/NetherEx/wiki/configuration#wiki_premade_biome_lists

Problem solved, thanks!