Problem solved!

I was wondering why the last chapter was suddenly numbered "20" whereas it used to be numbered "1" and while the two just before were numbered "1" and "2".
I tried again a trick I already did before:

1. I applied to both faulty titles the default paragraph style. So they disappeared from the Document Map.
2. I deleted the Section Breaks just before them.
3. I created Sections Breaks again.
4. I re-applied the Heading 1 style to the titles.

And voilà. It worked that time! The downside is that the underlying Heading 2 and Heading 3 titles are not corrected and are still numbered "1.1", "1.1.1" and so on so I have to apply the same thing to all of them and i have about 40 of them.

Let's hope it will last! I am now able to send my document.

Regards

Nick