Steps to reproduce: Add a speed effect to a clip, increase speed to 200%, click menu button on speed effect, click save effect, name it myspeed, delete speed effect from clip, right click on clip, select add effect -> custom -> myspeed. In the lower left corner of the kdenlive window, a message will read "Problem adding effect to clip".
This no longer relevant as the concept of changing the speed of a clip has changed with the timeline rewrite.