I have tried four different DVDs now. I have the problem at everybody.
Under Preview Menu -> Automatically start Preview Playback...is switched on.
The preview for the cell in question, doesn't work in the PGC Editor -> cell list.
The preview works for any cell, which has a seamless flag(8).
I start the preview for cell 19 (cell type flag=2 and LB), preview window opens -> no preview playback here and the functions above are inactive. (look at the snapshot)
In the Burn DVD dialog, it is just the same. Preview window opens -> no preview playback here and the functions above are inactive.
I go backward tomorrow ... beta 15 -> beta14->beta13 etc.. and then wanting to test.