I tested it now on 7.4.1708 and got the same error, maybe something changed from ntop version 2 to 3 but this is not intended I think. The ntopng application is working but Reports/Bandwidth Usage shows this, when authentication is enabled:
After a reboot the already changed password for ntopng admin was reset to default admin.
This one is already raised:
I think you found a bug. Thank you.