Hi all..
i found the trick with connecting the Boss FV-500L to the M5 for expression use.
i soldered a cable with one mono to stereo connectors. on the mono side i soldered the "ring" to the tip.The cable is like this
Stereo side: <Tip=Ring=ground==--------------==Ground=Ring&Tip> :Mono side
With this cable the expression pedal works fine.