Okay, I’ve checked it and found a problem.
If you go to “WPML – Languages”, under “Language URL format” you will see the following:
“It looks like languages per directories will not function. Details”
When you click “Details”, you get:
It looks like languages per directories will not function. Details
This can be a result of either:
WordPress is installed in a directory (not root) and you’re using default links.
URL rewriting is not enabled in your web server.
The web server cannot write to the .htaccess file
How to fix
When WPML accesses http://cuna.bwmedia.be/en/?____icl_validate_domain=1 it gets:
HTTP code: 404 (Not Found)
The expected value is:
It seems that the solution is when you click on “How to fix” link, https://wpml.org/?page_id=1010.
This is a WPML and WordPress issue.