like VLC media player, AMD Software: Adrenaline Edition, aquasuite (hardware monitoring with overlay), Microsoft Excel (only at start), but not with Google Chrome, Firefox, Notepad.
Of course, these programs didn't run at the same time as a game when there was only one screen. That explains why I didn't notice it earlier.
- or multiple instances of a game like Guild Wars.
Underclocking (via Automatic Tuning), as recommended in the linked post, doesn't help.
When recording the second screen, it turned out that recording (instead of a game) and running a program also causes this behavior.
I used the AMD Bug Report Tool, but never got an answer. Neither to my current request nor to my report about the incorrect labeling (in german) of the AMD Overlay.