I have already found the solution
...anyway, I hope this will help you to improve next release installation instructions ;)
When I first installed moodle, we only wanted to give it a first try, without worring too much about correct and secure installation. When we found it really interesting, I reviewed the installation instruction, copied the htaccess file of the lib subdirectory and edited the file size.
Now... it happens that on our installation (Apache 1x, PHP4...) the values given to the variables in lib/htaccess don't work, because they are expressed in '1' and '0'. Our installation needs these values to be 'on' or 'off' in order to work. It took some time for us to try this...
So it would be good idea to lead the attention of the installer to this detail in the installation instructions ;)