In the meantime I did a complete re-install and now everything work like expected. So it seems that the first installer I used (7.0.17) did not created required folders and file system rights in the right way. So after the deletion of Oruxmaps and the related folders I installed 7.1.2 from scratch. This time I copied the DEM files after getting Mapsforg themes to work - perhaps this did the trick?
From my point of view the issue is solved!
Thank you for your support and the greatest map app ever!