Thanks for your answer, sorry for not being clear. It happens when I edit the content of a page. Of any page actually. I've included two pics.
Now why would I want to type this it is because it is part of an URL I wanted to keep in my wiki. At first I included the whole "http://xxx.cd/xxx.php" and it kept saying the same error message as shown in the pic. And I removed all other parts of the URL and came to the conclusion that only the ".cd/" was buggy.
And it was not buggy a few versions ago because I have a few of these "http://xxx.cd/xxx" in my page that were saved successfully.