Hi Elisa, I just tested importing http://fr.flossmanuals.net/initiation-inkscape/ to the demo site and it did not work, so I can confirm the issue. What I would suggest is that you install Booktype 1.6.1 on a test server and start by looking for any errors in the log when you try an import.
If you take a look at the file scripts/createbooki in the Booktype 1.6.1 tarball, you will see that the error log is set according to the web server in use, for example:
/var/log/apache2/booktype-error.log
if you are using Apache. Let us know how you get on!
I noticed the same problem couple of days ago and I have identified problem for this. Not sure when the fix would come for this.
For upgrading you don't need to export and import books again. You should do backup of database, project directory (including attachments and etc) and then upgrade. This way you would also keep book history, usernames and all the other needed information. If you want some extra security you can always write a script which will dump each book into booki zip or epub file.
You would need to get new Booktype source, check all settings and modify if something needed, check if all the Django apps are installed, check all of your custom templates that they have correct paths and using good template tags. Do the dump of database and copy project files to test server and try to do upgrade there. This would help you recognise all possible issues which might come considering you are upgrading from very old version.