I did a green screen scene and applied select color and keylight 1.2 effect on the clip. When i screw for and backwards on the timeline i can see some stripes on the footage. They only go away when i change keylight effect off. I get this on every other clip with keylight too. The layer under the greenscreen footage is seen on that stripes also. The stripes are sometimes twice, sometimes 3 at the same time, but always only one frame.
Did somebody had this problem before?
Without knowing anything about your settings for the effect, the comp and details about the source files nobody can tell you much. Usually such errors indicate compression issues in the sources, where the decompressed and reconstructed frames have minute differences across their chroma info, because it has been built from incomplete data. This throws off color sensitive effects such as a keyer. Converting the sources to another format may help.
Try transcoding the original footage to a production format. An image sequence would be a good place to start.