Is it normal that most of the interactions like“hit on“ or „peck“ don‘t work at all? For example even if they can already make out I can‘t any flirt interaction, they just won‘t do anything. A lot of interactions just don‘t work.

Also isn‘t the whole point of the mod that they don‘t need to have feelings for each other to flirt/woohoo? Why do the sims still need to have 100 relationship points and be in love to woohoo, is that also the mod malfunctioning?

ACR has code that checks if someone would catch them cheating, and has them automatically reject interactions that would count as cheating if that's the case. That's the usual cause of that. If you have set ACR jealousy levels properly, it shouldn't be an issue, but it might be if you're using something else to control jealousy.

They were alone most of the time. ACR clashes with other jealousy mods? I have a mod that deactivates jealousy

And there were no other sims on the lot?

Another mod that isn't ACR that modifies jealousy will cause this, as I previously stated. If you use ACR, it's best to just use ACR for jealousy. I don't think there is any other jealousy mod out there that does something ACR can't do.