a) Install MPextended 0.6beta on the MediaPortal Server
b) Install the APP on the mobile devices
... I got stuttering video playback und sometimes no playback at all.
a) Replace the StreamingProfiles.xml with the newer version on the Mediaportal server.
b) Switched to Profile "SD-quality Android profile based on ffmpeg..." on each mobile device (settings).
c) Switched to "VLC player" on each mobile device (need to install the app first, settings).
Works great on 5 differed and rather old android devices now.