Hello Andrea, thanks for replying quickly.
I finally found the problem after reviewing every lines of code (not a big project anyway). I had a custom user profile property field in the DEV environment not found in the PROD; winded up not using it after all and it was not reference in the userModel property assigned… but since it exists in the class definition, the json initialized does refer to it. So DEV was happy, but PROD was getting a request with an unknown property reference.
Little details…