Nested sequence suddenly turns black

Here is my problem. I have two sequences. These are on two separate tracks and overlap slightly with a masked transition. The playback works fine. However, I want to add a little speedramp to the transition, so I nested the two clips. The newly interleaved sequence also plays back without any problems. But now the problem starts. If I trim the nested sequence or even if I just move it on the timeline, half of the sequence suddenly turns black/transparent.

I have had this problem on and off for over a year now. This over several PremierePro versions and on different systems PC/MAC.