hub3 reload of import failed

hi - just received a new hub3 to replace a faulty one.

I checked that the software version on both hubs exactly matched ... so I backed up the config on the old hub3 (admin Backup configuration settings), but when I try to restore that config to the new hub3 (using Reload configuration settings option of admin menu) I just get an error "IMPORT FAILED: File not recognised"....

As I'd spent time over last couple of years setting up old hub3 with guest network, and other config changes to the rules, I'd rather not have to start again.

Is this a bug or am I doing something wrong - thanks