Depending what version of hyperflex version you have, the HX gui stopped allowing the use of 'root' to log in.
Have you tried the account called 'admin' ?
What version of hyperflex is running?
Also, have you tried logging into the Hyperflex gui with administrator@vsphere.local account?
Also try using the username format of 'local/admin' or 'local/root'
Thanks,
Kirk...