The actual links in the paging when the number of questions is greater than 20 so the paging is working are defined as something like
notice the double /moodle_head_new/moodle_head_new/
this happens because the
add $CFG->wwwroot to the $pageurl which has been defined
$thispageurl = new moodle_url();
so the default path automatically generated is $ME
line 306 in weblib
in the example case the value is
So the $url to create the $paging bar should be different than the one used elsewhere in the showbank.
A simple HACK replacing in question/editlib.php
line 1275 $pagingbar = moodle_paging_bar::make($totalnumber, $page, $perpage, $pageurl);
$pageing_url = new moodle_url('edit.php');
$r = $pageing_url->params($pageurl->params());
$pagingbar = moodle_paging_bar::make($totalnumber, $page, $perpage, $pageing_url);
solve the problem either with the general question showbank and the showbank version visible when editing the quiz.
This happens as in either case the url goes to either question/edit.php or quiz/edit.php
These are the only 2 calls to this function.