I understood it now. The adjacent city Schönberg is a minor city 3 and the hex I placed the base belongs to it, too.

Are you sure? Because a valid space was the south western hex from Adler. I had troops there, too.

In the scenario about St. Vith I couldn't have placed a minor supply base in the city Adler:
The city has VP, has my units in it and contains a road.

What other requirements must be met?

I won it two times (normal and challenging difficulty).

My strategy in both games was to keep all western and southern roads closed and repel any breakout attempts while attacking from the north and the east. The airborne division in the south must hold against the allies reinforcements long enough.

I won it two times (normal and challenging difficulty).

My strategy in both games was to keep all western and southern roads closed and repel any breakout attempts while attacking from the north and the east. The airborne division in the south must hold against the allies reinforcements long enough.