As the wiring from the SolarCharger to the battery is too thin, I installed a BMV712 and replaced the MPPT 75/15 with a newer version that supports SmartNetworking.
Normally this works fine, the voltage is updated constantly. But as soon as I have more then ~7A flowing to the battery, the signal start dropping for an interval of 15-30 secs. The higher the current, the longer the dropouts.
In this time the charger works with its terminal voltage, and will switch from Bulk to Absorption.