Timm, it appears to be board- and accelerator specific. mbruines reported that the old ACA1233 works fine, while the ACA1233n doesn't, and it's worse on one main board than on another. If you want to replicate the problem, you should use the ACA1233n-55. IHave sent one of these accelerators to Peter as well, hoping that the problem shows on his machine.
I can confirm that this demo does have problems with an ACA1233n-55 here, and I saw black screens and the OSD several times. Also there were interruptions in the sound. Just for the record, when watching this demo on an ACA1233n-55 with native video/audio, it's not free of a plethora of other problems, and it crashes early.