@MIRKOSOFT: Hm, I have also tried it on my machines, and it looks like it is a general problem. (On my machines, the VDC is not detected at all.) Probably the VDC is not initialized by the operating system in this case, and this is why it behaves so unreliably. (I have only seen this issue with the IDE64 before, and thought that it is a hardware conflict caused by the IDE64 card, but it rather seems now that it is just because that card also forces the machine to directly boot into the C64 mode.)
My other test program (called as "VDC Dump") also fails this way. Thank you for spotting it again.
Now I need to write yet another little tool (I will call it "VDC Init") for "manually" initializing the VDC chip from the C64 mode, and that should be first run, still before running any of the test programs.
I don't know how to do it at the moment, but I am looking for some disassembly of the C128 mode Kernal, to repeat the same steps as the system does.
Once it is written, I will add it to the package, too.