[vikingthor]

Sound like your clipping the input of the Master. I noticed that it clips sooner than the output it self. Maybe something is going on in the groups.... ?As Empuler suggested

I noticed it a few times, most of the times there was plenty of headroom on the masterbus output. Just
driving the summing bus to loud. proper ( fader, gain and group) levels shout solve this, if this is the problem...