Thank you very much, Ofi, for the time you spent.
Good news: I finally solved the issue!
For a reason I can't explain, the process you described and that we both did didn't work on my side...
That's when I realised that the only difference between the two of us was the language of the interface! So I tested it with connecting myself to Joomla admin panel in English, did the process, and... it worked!!
Well I can't explain that, but now I know what to do.
And again, thanks a lot for your patience.