Ubuntu – iPhone mount errors

Have you tried:

sudo apt-get install libimobiledevice-utils;idevicepair unpair && idevicepair pair

Support for Apple devices under Linux is a long standing problem, which you can follow here: What is the current state of apple device (iPhone, iPod, etc.) support?

The specific question about accessing (and transferring) user files (pictures, videos, documents, etc.) which was possible until recently, but which is no longer after latest iOS updates, seems to require a new update to the libimobiledevice package which can be followed here: https://bugs.launchpad.net/ubuntu/+source/libimobiledevice/+bug/1623666 And clicking https://bugs.launchpad.net/ubuntu/+source/libimobiledevice/+bug/1623666/+affectsmetoo will allow the user to keep informed about developments in this regard.

Meanwhile, there are a number of workarounds (e.g. see the above-mentioned bug report discussions) enabling users to implement a temporary solution, which might be a little less user-friendly...