Thanks guys for your suggestions.
Meanwhile I "kind of fixed it" so now the situation is no longer horrible but VERY bizarre.
The good news is that it works again and I know what I did so it's all repeatable. The bad news is that it makes absloutely no sense (and I know what I mean, I have my daily share of software that behaves inexplicably)
So... after dismissing the scenery possibility, I focused on the module DLLs. And I focused brutally, by moving these (a little randomly, I admit) to a brand new "disabled" subfolder. Of course this made FS9 mad, but at least I got a different behaviour (huge step forward).
By following FS9 complaints about missing DLLs I relocated these one by one to the "Modules" folder until there was no more error message... and FS9 loaded successfully. I thought my culprit was one of the 6 DLLs remaining "disabled", so I "enabled" these one by one. And every time, FS9 started normally.
Eventually, I also re-activated the scenery (thus bringing FS9 in the VERY SAME configuration as the faulty one) and now everything works fine.
So here I am, happy flying but very, very puzzled.
Andrei
P.S. If anyone wonders whether I tried restarting the computer while FS9 was not working, to see what it gives, let me say OF COURSE I shut the thing down, I am a professional or what?