I have the exact same installation, everytime I switch off the mains to test if battery will keep the lights on, the inverter shuts down. I tried leaving only one light on and the inverter still shuts down. The two errors I get are 26 and 27 which are battery open and battery voltage low.
When I check online I noticed the error 27 frequently coming up.
Any advice on what could be the cause of these will be appreciated. I have been speaking to other people in the industry and so far I haven't been able to get to the bottom of this issue.