Mavlink not usable on /dev/ttyACM0

today I tested the latest version of PX4 on my F4Discovery board, but I came across some issues connecting it to QGC.
The startup script (rcS) runs fine and mavlink should be started in there.
But executing mavlink status after boot shows this:
nsh> [Kmavlink status

instance #0:
mavlink chan: #0
no telem status.
flow control:ON
tx: 0.194 kB/s
txerr: 0.000 kB/s
rx: 0.000 kB/s
rate mult: 0.679
accepting commands: YES
MAVLink version: 1
transport protocol: serial (/dev/ttyS1 @57600)

So I tried to start mavlink manually:
nsh> [Kmavlink start -d /dev/ttyACM0
INFO [mavlink] mode: Normal, data rate: 2880 B/s on /dev/ttyACM0 @ 57600B
WARN [mavlink] could not open /dev/ttyACM0

This fails and leads to the same result as above, when asking mavlink for its status.
The mavlink connection via ttyS1 to QGC works fine.