After exchanging a few emails with A2hosting, they were unable to help me with the matter.
Thanks for your help Ken, but I just don't have the skills to do those things confidently right now.
So, I restored a Softaculous backup of my 3.3.2 Moodle, and things are working (almost) fine, again.
I'm thinking of backing up my courses and installing a new Moodle, and restoring them there. It will probably take more time, but it might be easier and "cleaner" in the end. :/
Yann D.