@stefanleinberg the only other thing you can try is the flex cable coming from the sub board to the main board. Those have a tendency of failing. Ultimately it does sound like you are having issues with the power management IC for your charging circuit. I know you mentioned that you are not yet into board repairs etc, but here is what you could check
[image|3446308]
To locate the components get the [document|39740] and use the search function of your PDF reader to locate the components (i.e. C6001 and so on)