Question: Seemingly correct login but login screen reappears
I have an iMac about 2012. It had Sierra and all was fine. In a «momentary Lapse of reason» i attempted to upgrade to High Sierra. That went wrong. I have now managed to install El Capitan from Apple via <Apple>-R. So all looks normal - the login screen comes up with the available users. I type in username and password - it seems to be accepted but after a second or two the logon screen comes back. If i type an incorrect password the login is denied in the normal way.
So the users seem to be ok.
Anybody any idea what the reason for this behaviour could be?
iMac, OS X El Capitan (10.11.6)