In fact it's all my fault : I asked Kovid (in
this ticket) to change the behavior of calibredb so that we would not have to manually convert %28 into '(' and %29 into ')'
And then the change he made disabled urlencoding of the links in the stanza.xml file, which is overkill (but I should have said it beforehand).
He couldn't test it, because he would need an iPhone with Stanza...