[2020] The game said “you passed the maximum thrall number (30)” after i returned to my solo coop game for a long time. I removed the tick near “Use Minion Population Limit” in the settings. Can i now have more than 30 thralls? Im not sure if i unticked the right option box and want to be sure.

That should be the option, yes.

You can also change the limit of that cap with the sliders below that box (Thrall and Pet population base value and per player) in case you'd still want to have the cap in place.