About One-way streets, narrow single carriageways, separated carriageways and service roads: which statement is WRONG?

A sign usually indicates which carriageway to follow; if there are no signs, you must keep right and therefore follow the right-hand carriageway.
The prohibition applies to reversing as a driving manoeuvre along the road and to U-turns; backing into a parking space is explicitly still allowed.
Watch out for a sharp edge and/or a lower-lying verge on narrow single carriageways outside the built-up area; if your right-hand wheels leave the road, you quickly lose control of your steering.
✓ A service road (ventweg) next to the main carriageway is simply an extra lane you can use for fast, through traffic.

Correct answer: A service road (ventweg) next to the main carriageway is simply an extra lane you can use for fast, through traffic.

Explanation

The main road is intended for fast and through traffic, the ventweg for slow and local traffic (sign C5 at the entrance)

A sign usually indicates which carriageway to follow; if there are no signs, you must keep right and therefore follow the right-hand carriageway
The prohibition applies to reversing as a driving manoeuvre along the road and to U-turns; backing into a parking space is explicitly still allowed
Watch out for a sharp edge and/or a lower-lying verge on narrow single carriageways outside the built-up area; if your right-hand wheels leave the road, you quickly lose control of your steering