What vehicles may enter a zone with this sign?

Your answer is fully correct.

This traffic sign indicates a bicycle zone ("Radfahrzone") where primarily bicycles are allowed. The additional sign "Anlieger frei" means that residents or those with legitimate business ("Anlieger") are also permitted to enter the zone.

In detail:
1. Bicycles are always allowed to enter a bicycle zone.
2. Small electric vehicles (such as electric scooters) are generally permitted in such zones unless specifically prohibited.
3. Vehicles driven by residents or those with valid reasons (like deliveries, visitors, or property access) may enter despite the restriction.

The purpose of this sign combination is to limit traffic to mainly bicycles and allow only essential motor vehicle access, reducing through traffic and enhancing safety for cyclists.