You was right. I've read the documentation (RTFM) here
and I've learned the following..
The OSE edition misses these features (in comparison to the binary edition):
- The virtual USB 2.0 (EHCI) device; see the section called “USB settings”.
- VirtualBox Remote Desktop Protocol (VRDP) support; see the section called “Remote display (VRDP support)”.
- Intel PXE boot ROM with support for the E1000 network card.
- Experimental support for PCI passthrough on Linux hosts; see the section called “PCI passthrough”.
These feature can be added to the OSE edition installing the VirtualBox Extension Pack from here
So I have installed the extension pack .. and now I have RDP working :-)
Thank you for the hint