Hi again!
Although I was ready to give up, I carefully read the support page that Chris posted above (https://support.fbsocialreader.com/customer/portal/articles/616021-reads-aren-t-being-published-to-facebook) and I decided to change my permalink structure (as STEP 4 in support page suggests), as I’ve been using the “default” WordPress permalink settings.
So, I changed the permalink common settings to “numeric“. And it worked! ?? I managed to submit the app and now it’s in “pending” phase.
The thing is that I’m not sure of the mess I may have caused to the mySQL by that permalink change, but I gave it a try anyway… I’ll let you know. ??