Mixamo destroys fbx model

Dear community,

after i spent half a day trying to solve this issue, i need your support.

I want to animate a custom character (.fbx) via Mixamo. In the preview, the model looks fine, also with animations. But after i download, the .fbx file it looks like the mashes get totally mixed up. 

This is the character in the preview:

And this is how Mixamo exports it:

Any suggestions what might be wrong?

Thanks and best regards