Preview render hasn't the same lightning as the final render.

Very confusing. Haven’t had this before.

Preview render looks good. Final render is way too bright.
Haven’t done anything special. Except for AO. Never touched it before. Played around a bit with it and turned it off. Can it be due to this?

Hm, went out of preview render and back in and now the lightning matches the final render.

Yes it’s AO or else switch to blender render go to render settings/shading and check Color Management, then switch back to cycles and you’ll be ok.