This error occurs when the internal bus voltage exceeds the 470Volts.
Possible cause: both the Solar and Battery module are trying to create/maintain the bus.
This causes the voltage to increase rapidly resulting in this error.
Solution: Normally the PowerRouter should be able to solve this issue automatically.
If it keeps failing, a possible solution is to switch off solar (with the DC switch) and restart the Powerrouter (Menu, service, restart). After the PowerRouter has started, wait 1 minute and switch on solar.