Handle external urls in topic body for subfolder installation

Hi, guys!

We have forum installed in subfolder: https://i.materialise.com/forum/
It seems I have similar problem with external links. When user clicks on external link, he gets redirect to our domain with path and query string from original url.
For example:
https://i.materialise.com/watch?v=<video_id>
instead of
https://youtube.com/watch?v=<video_id>

Open in new tab menu and middle button click works well.

Please advise, how to solve this.