I had this same issue nowadays while running XWiki in Kubernetes.
My solution was to create the admin user directly in the wiki config file (/usr/local/xwiki/data/xwiki.cfg in my case, maybe /etc/xwiki/xwiki.cfg in yours) like:
xwiki.superadminpassword=yourpassword
Then restart the service (or redeploy the pod if Kubernetes), then access the application using:
login: superadmin
password: yourpassword
Now you can follow the setup wizard.
Hope it helps somebody. 