Yes, there were (more or less successful) attempts made at filming those books. Due to their large popularity in Germany there were two recent film adaptations made as German/Austrian/international productions, adapting two of the original novels (according to Wikipedia with varying degrees of faithfulness to the book stories, though):
According to the German Wikipedia article the production studio that bought the film rights (Studio Hamburg) planned to initiate a broad merchandising campaign, including the above and more theatrical movies and a planned TV-show. But after what seemed to be internal personell struggles in the studio, it said that this plan had been put on the test bench again, which put the planned third part in the movie series (supposedly adapting The Mystery of The Silver Spider) in limbo, albeit assurances by the studio that the film is in production.
I can't say much about the actual filming rights situation and about any other international or English-language adaptations that might be planned or have been done. At least Wikipedia doesn't say much more on the matter. The above linked German Wikipedia article does say that Studio Hamburg bought the filming rights, but I don't know if that excludes anyone else from adapting the series and how such matters are handled on an international scale, especially since the situation with the books is unusual in the first place, seeing that after the cancelation of the original American book series German authors kept on publishing new stories due to the series' large popularity there.