Here is a video what contains similar parts as yours:
Sheetmetal material thickness should be equivalent and you have to be in Ordered mode!
Or use what was suggested by @MattJohnsonPAC!
Thanks all of you for your help!
With the command flattern of a part I "just" get a surface? Because I want to work (bevelling) at the unrolled part and bend it again.
For the moment I´ll try to create my modell as a sheet metall (only with one thickness) , so I think your video will help me a lot.
At the moment I´m not able to upload my part, but I will try later.