Sorry, Moritz! I could not find this source again among dozens or hundreds od magazines and books. I have seen only reports about Stralsund and Johannisthal. Therefore you should contact Jet&Prop via e-Mail or letter. I made good experiences with this method.
Jet&Prop:
www.vpm.de/vdm/jet&prop/jet&prop.htm
Fliegerrevue (NOT Flugrevue):
www.fliegerrevue.de
You could also contact some Eastgerman aviation enthusiasts like Thomas Krafft (he has a nice MiG-21 site) or Hagen Glock (both former members of of the Eastgerman unit Jagdgeschwader 8).