Sorry, at the moment i'm a little bit busy, but fixed this issue (same description, but differents test) - thanks for your solution and comments;-)!

Found the issue: I didn't add spaces in the result.
After adding spaces to the result string, I could pass the tests.
Since the explanation explicitly states that these spaces aren't required:

Spaces in your result are allowed, but not necessary.

This should either be changed in the explanation, or in the tests!