I mean when you go to the page 2 your sorting breaks
here is your code of paginator
<div class="pagination"><span class="XY">Seite 1 von 2</span><span class="current">1</span><a href="http://www.naehmaschinekaufen.de/websites/new/wordpress/alle-tests/page/2/" class="inactive">2</a></div>
it should be
for example
<div class="pagination"><span class="XY">Seite 1 von 2</span><span class="current">1</span><a href="http://www.naehmaschinekaufen.de/websites/new/wordpress/alle-tests/page/2/?sort=preis&order=asc" class="inactive">2</a></div>
when you generate page links add ordering to them
