Update: I managed to fix it, the truth is very rare.
The first thing I did delete from forge that I had installed (36.2.0 / 362.5); then I reinstalled the launcher (TLauncher); then I installed forge with a new "mods" folder, and that is empty; and in the end I added all the mods back to the new empty "mods" folder.
And voila, the demo mode has stopped appearing for me.