What vehicles may enter a cycle highway that has this sign?

Your answer is correct. The sign "Fahrradstraße" designates a cycle street or cycle highway where bicycles have priority. The additional sign "Anlieger frei" means that residents or those with a legitimate reason to be there (such as property owners or those accessing businesses) are allowed to enter with other types of vehicles besides bicycles.

According to German traffic rules, on a Fahrradstraße only bicycles may generally use the road, but exceptions are made for residents or local traffic ("Anlieger"). This helps maintain safety and priority for cyclists while allowing limited motor vehicle access for necessary purposes. Other vehicles like mopeds under 25 km/h or cars at walking speed are not automatically allowed unless they are residents or local traffic as stated by the sign.

In summary, the rule is: non-bicycle vehicles may only enter a cycle highway if they fall under the "Anlieger frei" exception, which means local residents or users with special access rights. This ensures cycling remains the main priority on such roads.