Going my way (hope it helps someone …)
In fact, the problem is only in forms.
I could not be aware of it as I could not login.
I managed to log in after re-installing cake 3.3 and once logged in, I could re-activate cake 3.5 and everything works fine except forms.
So I cannot add or edit anything and I cannot log in either.
Still working on it.
If somebody can help me, he’s welcome.