Everything worked perfectly fine for a quite a while but suddenly the solar charger shows a voltage that's not accurate. It is still charging though.
SmartSolar detects 12,95V battery voltage at night, so it's only the battery voltage, no charging voltage (Standard AGM).
A reliable multimeter shows 12,72V measured directly at the connection screws of the solar charger, so it cannot be some corrupt cable or anything.
The 20A fuse is fine.
Load output works fine.
I don't know whether there was any firmware update before the problem started, earlier this year everything was fine. Firmware is v1.59 now.
I could just modify the charging parameters accordingly, but that doesn't seem to be the best solution.