convert song to sequence = garbage and mess?

Well,

i have made 4 sequences that I chained in a song. I thought that the 20 measure progression would make a nice intro, worth some fine editing. I thus converted the song to a new sequence...

the resulting sequence is full of garbage, all the programme assignements are fukced up (some have question marks in them) and the thing doesn't play properly.

The strangest thing is that I use the same programme assignment on all used tracks in all relevant sequences, i.e.:
track 1 uses programme 1
tracks 2 and 3 use programme 3
track 5 uses programme 5
track 10 and 11 use programme 6

Any suggestions on what to look up first?