qTranslate Plus – Weird reset of get variables/Redirect

I have this weird issue at my website where wordpress deletes a get variable/redirects to another link.

I am using the plugin qTranslate Plus (slug is xtranslate-xp).
The issue can be best demonstrated with an example:

The issue can be reproduced on this page: https://www.leo-d-roorda.nl/en/vl35/
If you click for example on the link “Climbing Stairs Questionnaire (CSQ15)” (linking to https://www.leo-d-roorda.nl/en/vt15), the issue will occur.

What happens is the following: The link opens, and right after it the page redirects to https://www.leo-d-roorda.nl/vt15, which has lost the /en/ part in the url.

You don’t have to be a rocket scientist to understand that if the /en/ part in the url has been lost the language has also been lost.

Is there anyone who knows what could be the issue of this happening? I get the feeling this is not a plugin related issue, but I’m not sure.

An interesting fact:
Once on the page itself, it is possible to go to the English version by adding /en/ to the url or by clicking the flag.

Another interesting fact:
Once on an English page, it is possible to go to other English pages by using the menu.

Summarizing:
Using the menu/flags is okay, however, the links on the page themselves don’t work.