First of to avoid confusion I use the US GOTY version of Oblivion with all DLC. However since I'm German, I also use a lot of German mods next to my english mods. It's mostly house, beauty, race and quest mods. It's the quest mods especially that have been causing me trouble. Many of the German quest mods like Adash the city of Magic, Alt Bravil, die Rache des Ketzers, der Orden des Drachen seem to clash with english mods ike Dark Brotherhood Chronicles or the Lost Spires. The problem is that with most mods there is a list which mods it will clash with and which mods it won't, but that only applies to mods from the same language, so my approximation of which mods will work out and which won't is limited. For example I couldn't finish the normal Dark Brotherhood questline, because I couldn't kill Ungolim during the last questline. Something. most likely one of the quests, had made him an essential character. I tried turning it off via the console then mods, but nothing worked until I disabled all my mods one by one, which caused the whole game to crash. I had to uninstall the whole game and reinstall it just to play one quest.
I'm still a newb when it comes to modding, can someone with a bit more experience give me some tips in how avoid this in the future.