Then, I've put in a vocal channel (mono mic from my Mackie Firewire interface) on concert level.
Some stage must be missing in your description because as far as I know it shouldn't be possible to open an audio input at concert level.
Now, all of a sudden, everything is MONO - Even after I've deleted the vocal channel, Mobius plays back mono only!
I hope you can help me out of this muddy mono feeling...
Have you checked that you are not by mistake sending the signal through a mono channel? Look at the channel strip, just under the fader: a zero icon means "mono", a horizontal "8" means "stereo".