How to reproduce:
Have a 2 video track above each other, transparent text clip and video clip
Everything is OK now
Insert an empty video track between text and video clip.
Transparency of text clip is lost! It's got a black background now.
Changing visibility of an empty track has no effect on it. You have to delete the middle empty track to get thing back to normal.
This is a serious bug.
Full kdenlive rebuild (kdenlive_builder script) from SVN sources updated 16.6.2007 (most recent ffmpeg+mlt+kdenlive at that time)