I had error 71 once. I put a 48v charger on the bus bar and turned off the 'good' batteries. Brought everything up to four lights, then let it go. I haven't had the same problem since, but have had full sun days in the meantime and have limited the load quite a bit.
I emailed SS, and their response was 'Fault 71 is a firmware version inconsistent. Each inverter needs to be updated to the same version.'
I decided to throw in the towel on the 6500ex and ordered the 18kPV. I struggled with the decision to buy any EG4 product, or switch to Schneider, Victron or others. I ordered my 6500's right at the end of the cycle. I had no idea I was ordering an obsolete product. Like buying a computer right before the next model comes out. I hope my solar future is less dramatic going forward. I just want something to work....and work well with minimal input from me.