I am also getting the same issue on a new array; Activated: Not up-to-date Failed to sync flash backup.
All plugins are up to date.
I tried Deactivate as well as Reinitialize options however nothing worked. I not seeing any error or logs, neither in unraid nor in router/firewall which could explain the ' Failed to sync flash backup' error.
Though local flash drive backup did worked and currently array is still calculating Parity, can someone please suggest what could be the issue?
Once parity calculation is completed, I am planning to reboot to check if that helps with this error.