My VAG-COM or the adapter seems to be malfunctioning somehow.
"With my Golf 4 (model year 2003), I can do anything I want – access control units, etc."
My friend has an A3 8P, and he can find the control units, but I'm unable to access them.
And if you take a closer look at Auto-Intern, you will realize that for the 8P, you need a CAN-compatible interface.
If your system works with the G IV but not the A3, you definitely don't have a CAN-compatible...
Our host also has the right items for the 8P model in his shop .
It has been said before, but the best option would be to contact the provider from whom you purchased your hardware. Besides that, what do you see when you run VAG-COM > Options > Test?