Hey Zoltan, thanks for the answer.
Unfortunately setting back the composition size does not fix this problem completely, because all clips also takes over the double resolution. So that is something I have to reset per clip .

Hello!
I am using Arena 7 on a Mac and a Windows computer.
I created a composition with a resolution of 3840 x 2160.
When I open the file on my Windows computer it has the double resolution (7680 x 4320)
Why is that?
Thanks for help,
Hendrik