Ok. So I have been running my Synapse 7.2 with Pylontech UP5000 for almost a year now, after also had to make custom rs485 cable. Installed PVs last week and played with some charging priority settings. After doing a full on inverter "reboot" this morning, the inverter displayed the dreadful 61 error. As solution, I changed the baud rate dipswitch on the pylontech to "9600 CAN resistance: connected". On my UP5000, it was dip1=on, with dip2 to dip4 all set to off.
It now still seems to be working as expected; Still no idea why it went into limbo. Anyway, hope this helps.
Johan