In the end, this suggestion found here did the trick:

  Run the registry editor (type in “regedit” in the command-line”)
  Backup your current registry (File -> Export)
  Navigate the following path: HKEY_LOCAL_MACHINE / SYSTEM / CurrentControlSet / Control / Class
  On the left side, search for the entry {EEC5AD98-8080-425F-922A-DABF3DE3F69A} and click on it
  On the right side, you will find a key which begins with “Upper…” – delete it!
  Now unplug your iPhone
  Restart your computer
  Plug in your iPhone and hopefully find it mounted as a drive again!

ist durch langes Drücken auf „&“ zu finden