Black frames exporting Quick Time

[SuperHenning]

I´m making some film for a web-site in different sizes. I have been using After Effect for 11 years, but never seen this. When I do the export, whith the same settings, it put big black frames on the site in some of my exports.  If I export as PNG , there is no problem. The are no black in my sequence. What to do? 

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[SuperHenning]

The 2 exports are 600x3000 black frames, 600x1500 no black frame

[Szalam]

There's not enough information here to diagnose much with. What is the other export setting you're doing? Is it the same composition just with different render settings?