Without any info about actual render settings and what the layers look like in the comp and timeline nobody can tell you much. From effects producing a different look at different bit depths to issues with hardware accelerated processing messing up things this could be anything. You have to be more specific.
That is a media player. Media players do not support transparency. To check for transparency you need to open the file in After Effects or Premiere Pro.
* The "Drag & Drop here..." area is buggy and should not be used to share images. Please use the toolbar or just drag your images to the reply field.
problem solved trikmat i choose alpha instead of matte