That doesn't make any sense to be honest, I have no idea why that could be happening unless you're attacking cities that belong to those lords, but that can't be true for every single one of them.

For a fix, there is no way to do it quickly without cheating. Don't feel bad about cheating in warband, especially warband mods. Lol Its a buggy game and this mod in particular is pretty buggy itself. Like I said I've never heard of this happening in this mod or any other warband mod for that matter. Pretty bizarre.

My advice, start over. Make a new character, export it, level him up to where your last character was at, then import him and start as a vassal or faction leader. It will be easier than trying to drag your reputation out of the negatives with a dozen lords.

I think it would be faster to make a new character where your old one left off than sit and try to reverse the damage.

Ok, thank you.
But just in order not to make such a mistake in the future, what do I have to do?
It's just every time i conquer a city, one or two generals get positive reputation because i give the city to them/their relative, but other 10 or so get pissed off. I tried managing it, but the overall reputation losses are greater than gains every time.
Is it somehow tied to the country's military power? I tried to play British Colonies before, there were like 5 generals overall, and i nearly maxed relations with them. But now, with the German Empire, there are 5 that like me and 20 or more that hate me :(

You gotta wait for a general census to form before you give up a fief. Ask your lords "To whom would you want the fief to go" or something like that after taking a fief and every general will have their opinions. Note the name mentioned the most so you'll gain the most opinion and lose the least