What way does 20 AB cap work? I mean, as far as I know, ability bonus (say, strength) isn't a subject of the cap. But what bonuses do apply first? For example, my BAB is 11, my Str is 22, my enchant/feat bonuses are +5. So is AB +20 or +22?

This says different http://nwn2.wikia.com/wiki/Attack_bonus

Oooh... I never new that one. I very rarely play any high-level modules so it would have never affected me.

It work like this: your Bab and class feats like weapon focus or enhanced arrows is not subject to cup. Your natural stat bonus is also not subject to cup(it subject to separate cup, as far as I remember you stop getting bonuses if you reach natural 60 in stat). That means that fighter10/brd1/arcane archer 9 with 30 natural Dex and EWF will have 29+6+5+10=50 ab. And on top of this you can add 20 more ab from buffs and items.

Other stuff works toward 20 ab cup and then rounded down if it overflow it. Additionally rebuffs do not pull your cup higher, so if you for example buffed previous character to +20 ab, and then wear something like construct girdle(+12 str -6 dex), you won't be able to buff yourself for +3 more ab and net gain would be not +20, but +17