Headphone Output Problem?

Thanks for the fast return!

I use it as a stand alone recorder, connected to the direct outs of my mixing desk.
I sent a good leveled mic signal to the utrack.
The connections must be ok, as I receive a proper signal on the rear main outs and the level is perfect on the LEDs when soloing.
But with several headphones, I always get a very low, thin sounding signal on the front headphone out.
When soloing the incoming signal, I have to turn the headphone volume knob up completely to hear anything at all.
As the solo is “to-tape” I should be able to check / solo the signal through headphones without a harddrive connected, right?

Thanks!
Christoph