Check the connections between MPPT and battery, both positive and negative, including fuses. Also verify with a multimeter that both voltages visible in the APP are correct. Im sure theres a bad connection or a blown fuse. If theres no current flowing, they should both register the same voltage while they are connected.
Thanks, I checked everything multiple times, but I might replace the fuses just to be sure. So it should be the same voltage on Liontron (battery) app and victron-mppt-app when the connection is good? Are the different voltages an indicator of a broken connection?