Strange borders in stacks

Hi Mabula,

end of last December and the last 2 nights in January I captured the Pleiades in both luminance and RGB with my SBIG CCD and SW 80 mm APO. As usual I process the images in APP using the "Multi Channel Processing". In the integration tab at "Multichannel" I select "Integrate per channel & All". I really like this option because the integration of all the channels combined (L +R, G and B) gives a slightly better "luminance" stack then when I only stack the luminance frames.

I ran into a problem with this. When using the "Multichannel" option the stacks from R, G and B have some strange borders at the top and bottom. Strange thing is that when I process the frames individually (i.e. without the "Multichannel" function) and with the same settings, the borders are just fine. Even stranger is that the luminance stack doesn't have these borders. The combined stack (R, G, B and L) also shows these borders.

Attached you'll find 2 stacks from the B-channel: one stacked with the "Multichannel" function and the other without. Do you have any idea what causes this problem?