sorry, not sure what I am looking at there… might be since I cant read the language…
is the first image (and link) the main forum page? and the second one a forum page? thinking so, but want to check…
believe its because you are serving cached pages… see this in page source
<!– Performance optimized by W3 Total Cache. Learn more: http://www.w3-edge.com/wordpress-plugins/
Minified using disk
Page Caching using disk: enhanced
Database Caching 2/86 queries in 0.186 seconds using disk
Object Caching 3349/3412 objects using disk
Content Delivery Network via N/A
Served from: www.trylleskolen.dk @ 2015-10-10 15:32:48 by W3 Total Cache –>
you cannot cache the forum pages… each page is dynamic and shown on a single wp page… caching plugins fail to properly handle dynamic page generation… so you have to disable all the types of caching on the wp page the forum is dispalyed on… w3 total cache provides a mechanism for disabling the caching types by page slug, in your case, /forum