thank you, your tips helped finding the problem. But I'm still working on a solution...
Found out that the gpx file was corrupted while downloading from email. Was sent with about 380kb in size, arrived with the same size, and after downloading it was only 28byte, containing the text "Decoding Error".
So no problem with oruxmap, but one with my email. Any experts here???
Thank you!
Bye, Kai