variable @screen-xs-max is undefined in file /hosting/huisontruiming-regionaal.nl/public_html/lcms2/default_templates/formulier/nieuwsbrief/style.less in style.less on line 11, column 31
09|             display:inline-block;            
10|             vertical-align:middle;
11|             @media (max-width:@screen-xs-max){
12|                 width:70%;
13|                 vertical-align:bottom;
14|             }