Dear Manuel,
We just identified the root cause. Actually it seems to be related to the Android 6 upgrade as you assumed as well.
Generally it looks currently like an incompatibility between Android 6 and older versions of SAP Business One.
Please see SAP note 2315171 - Mobile application on Android 6.0 shows "Connection failed" error
In complement to an upgrade we still look for alternative solutions/workarounds. We will update the note in this regard asap.
Best regards, Peter
Message was edited by: Peter Hartwich