Oh, I see what's going on.
A "feature" to disable the navigation buttons after clicking has been added to 1.91. Unfortunately, this script destroys all workarounds that interrupt the Next/Submit function.
A quick fix until I can come up with something more elegant would be to override the "feature" by adding the following to the very end of
endpage.pstpl.
<script type="text/javascript" charset="utf-8">
$(document).ready(function() {
$('form#limesurvey').submit(function() {
$('input[type=button], input[type=submit]', this).attr('disabled', '');
});
});
</script>