SUMMARY Today I installed neon unstable based on ubuntu 20.04 and I can't start plasma with the password typed during install process. I can start plasma without a password but User kcm indicates that I'm using a default user account and I can't do anything that requires administrator privileges. I have downloaded neon-unstable-20200810-1849.iso
I can confirm this behavior with the same ISO file. I was able to perform a recovery boot, add a password for my user as well as adding it to the sudo group. Once I rebooted everything else seems to be working.
I think this was Calamares bug #1489. All ISOs were being fixed on Friday 21st Aug and I have seen no reported problems since (forum, reddit).