Dutch words are sometime longer than the English. They no longer fit in the column. How can I change the width of a column? Another strange issue (in my mind) is that when I change from the forum to an topic, the screen is changed in 75% of the width. My theme have no sidebar in it.

Examples, screens and URLs please. I can't follow you.

See the attachments:

You website content is too small, you should use forum w/o sidebars. Or you should increase website wrapper width.

Also you can use simple CSS code. Please add this CSS code in your current active theme's style.css file. Save it and do CTRL+F5 on front-end to load new content of CSS fiel:

#wpforo #wpforo-wrap .wpfl-1 .cat-stat-topics{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .cat-stat-posts{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .cat-title { width: 69%; }
#wpforo #wpforo-wrap .wpfl-1 .wpforo-forum-info{width: 60%;}
#wpforo #wpforo-wrap .wpfl-1 .wpforo-forum-stat-posts{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .wpforo-forum-stat-topics{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .head-stat-posts{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .head-stat-views{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .head-title{ width: 69%; }
#wpforo #wpforo-wrap .wpfl-1 .wpforo-topic-info{ width: 60%;}
#wpforo #wpforo-wrap .wpfl-1 .wpforo-topic-stat-views{ width: 15%; }
#wpforo #wpforo-wrap .wpfl-1 .wpforo-topic-stat-posts{ width: 15%; }

Thanks Robert, adding those CSS is the solution. Even with a forum with a sidebar.  I'm very satisfied with your forum. Much better than all the other forums that i have used in the past 10m years! And not alone the forum but also the support.