Finally found a solution...
- copy the dokuwiki installation in a new directory,
- download and extract Dokuwiki-2009-12-25c.tgz “Lemming” from splitbrain.org, the version I was already in,
- copy the content of the archive to my new directory,
- test (successfully) the "new" installation,
- rename the "old" installation and the "new" installation to use the working one...
OK, it works, but the solution I found does not suit me. Maybe the problem was something like
what is discussed in this thread, but it is frustrating that I did not find the reason...
I hope it will help somebody else one day or another ! ;)