Hello Gerben,
See if this helps.
Q1. Also as others have mentioned is it happening only behind TMG or Reverse proxy?, how is the situation if you try to connect the BO Server directly? Is the Design Studio getting listed.
Q2. Also what are the device specs, i mean which iPhone's or iPad's are being used.
Would recommend to upgrade to IOS 9 as IOS9 has a better Webkit support.
Q3. Also if you could test the latest version of the App in app store and let me know the results on the crash.
Regards
Vikas