My Sims don't respond to actions and if I try to cancel them the red cross won't disappear and my characters just stay there, clapping their hands or whatever sims usually do when they have nothing to do.
I've installed Late Night a few days ago but this problem just started yesterday or so.
I've tried re-starting the game but it doesn't work since my sims will sooner or later just "freeze".
The resetsim cheat doesn't work either since the cancelled action remains there.
I have the Extented Tunable Core but I've never had any problems with this mod.
Does anyone know how to fix this?

You need to go to the Help section and try the FAQs. If you still have questions, you'll find a link to the Help forum at the bottom of the page.

However, I can tell you now -- if your core mod has not been updated for LN, that is certainly going to cause problems. Not having problems in the past is no indication Most gameplay-affecting mods, core or not, need updating with every patch/EP, since they modify game code which changes with expansions.