I have the same problem. My video material multiplex m2v and ac3 5.1.
DVDLab pro have error Why?
Because audio and video material have differend timeline.
video have 2:12:54:102
Audio have 2:12:54:107
That`s the problem
You must again re-encode audio and video material together witch the same software Mainconcept,.... etc. Maybe one way is only stereo ac3 not 5.1
Or build new compilation witch other software sonic scanarist,pinacle studio,unlead videostudio,TM encoder,...