Well, my mistake, I was stupidly copying the doc, which is obviously (sometimes ?) wrong.
For the Query top element, do not use
xmlns:qkr="http://query.quickr.content.ibm.com/1.0" as proposed in this doc (http://www-10.lotus.com/ldd/lqwiki.nsf/dx/query-content
), but xmlns:qkr="urn:ibm.com/td/query" as proposed in another page of the doc.
As a principle, test all the variants proposed in all versions of all the pages of the doc. Combinations do not outstand the total number of atoms in the universe. That's the good news. Seriously, dealing with this API starts to be kind of time-costly. But, I have to acknowledge that this one is still better than the REST API for Domino.
Good luck readers !