Hello there! Thanks for your answers! Sry for my late reply, but i didn't expect any answer after weeks of no reply to my post.
So if i put my rendered video back in DR it looks good again, darker and more saturation.
On my Mac i use Quick time to compare the DR Preview with the rendered video. On my Windows pc i use VLC but also tried Quicktime, because i know that there are some differences in the colors between the players. But that's not the problem
I also unchecked the "use mac display for preview" but that didn't change anything.
The broadcast safe in the preferences is deactivated by default.
Like i said, the way i "fixed" the problem is to use my Windows PC for Colorgrading in DR. There i don't have to change anything, the preview does look excactly like the rendered video. I think that's how it should be. Im not a professional colorgrader, i was just searching for a programm to cut and color grade some videos for my youtube channel. Tried different programms like Adobe and Final cut, but for me DR is the easiest way to do it. According to the slogan "easy to use, difficult to master".
In conclusion its sad that i couldn't use my mac for this because of the problem. On Windows i have to convert every video with the help of imovie, otherwise DR isn't able to see the files.
btw sry for my english, i'm german