I ran into a split fabric issue setting up my test lab and got the following error trying to log into my 2nd APIC:
REST Endpoint user authorization datastore is not initialized – Check Fabric Membership Status of this fabric node
I was able to get logged into the APIC with the follow username and a blank password:
NOTE: as in the past physical access to a Cisco device equal total ownership.
Basically when installing the fabric for the 1st time you should only power on 1 APIC and discover the entire fabric, then add the other APICs 1 at a time.