Okay. After looking into the source code of the scraper, retrieving debug info and examining the RegExp entries under GetSearchResults in the tmdb.xml, I found that it should work. Then I found that there was an updated version of the add-on installed in my system. The original version on Leia is 5.1.7 and the update version is 5.3.0. The RegExp of this updated version did not work according to my tests. Leaving me to believe that an automatic update of the add-on was to blame. Removed the corresponding folders under ~/.kodi/addons and rebooted the system. Now the scraper works again. I do needed to update the two dependencies to get the thumbnails downloaded correctly.
I will also give the python version a try in the future.