So I manually installed the three unofficial patches for Oblivion. After enabling all the esps the game crashes when starting. Through process of elimination it seems the esp for the unofficial patch for Knights of the Nine DLC is the cause. Disabling it from the data files prevents the crash.

Seems like other people also have had this problem, even those who used mod managers i think.

Does anyone has any idea on how to solve this?
Perhaps I will just play without enabling it.

No 1 tip. Clean install of game.
No 2 tip. Use a mod manager and sort order tool.
https://loot.github.io/
https://boss-developers.github.io/
Mod organiser or Wrye bash.

Personally I've had more trouble with the Unofficial Patches for Oblivion such as texture corruption and missing textures in Shivering isles. They are not really necessary in my opinion if you just want to experience Oblivion vanilla. Maybe use it for base Oblivion but not for the expansions?

I recommend Wrye Bash for merging and taking care of mods/mod order. Great for installing, great for managing and removal. Requires a little bit of education to learn, but is great once you've properly figured it out.

I don't recall having any issues with the Unofficial Patches... but if you're having problems with one of them, you can just play without it, of course.

I strongly am against the use of BOSS, its very outdated and prone to falsifyable errors which may take hours to hunt down and fix for Oblivion related stuff. Always choose to use LOOT over BOSS instead, you won't have many problems with that program.

I recommend using Mod Organiser over Wyre Bash to steamline the modding process, however using only Wyre Bash for any Oblivion related needs is absolutely fine here as well. With Mod Organizer you can also easily link other modding tools, such as LOOT. The only downside is that the game will take longer to load.

At any case, put the Nights of the Nine file below the regular unofficial patch and you should be fine. Its 100% a clean instill/loadorder problem.