What are the minima for a non precision approach?
Depends on a lot of factors. In Egelsbach they probably would be very close to the german VFR minima in class G airspace under which they very operating anyway: 1500m visibility, clear of clouds and not below 500ft GND unless taking off or landing.
The slashes in the Metar replacing the values for visibility and clouds stand for "cannot be determined" which can mean anything. We will have to wait for reports from other pilots and witnesses on the ground what the met conditions were like at the time of the accident.