The website and API share the same search so you can use it as a test. As you can see this search works fine:

https://www.themoviedb.org/search?query=Rene+Marik+-+KasperPop

And on the API…

http://api.themoviedb.org/3/search/movie?api_key=###&query=Rene+Marik+-+KasperPop

It also returns it.

You'll have to contact XBMC to see what they are doing to the search to cause it to fail.

I tried it another time like 20 times before, now it worked. :-)

Thanks for your really quick answer.