Hi Stefan,
Yep, I viewed it on a Desktop.
Now I tested the .be website on my phone and I noticed the issue
The issue is caused by the way you do the redirection.
I don't know how you do it, I mean is it a Joomla extension, is it a feature that comes from your Hosting Provider, etc.
The thing is that the redirection is wrapping the whole website in additional
div
and then an additional
iframe
. And this is breaking the responsiveness.
Here, have a look at
this video that I recorded for you. It shows what I mean.
Just remove the current redirection and do a proper one

If you are not sure how to do it, just contact your Hosting Support and they should do it for you.
Apart from breaking the responsiveness, your current setup (redirection) is not good because your website can be accessed with two different URLs which for Google is duplicate content