After changing User Management settings I get logged out

Hi there, hope you can help.

I was trying out the user management settings and must have done something wrong. Every time I now try and go back into the user settings I am automatically logged out of the site (whether the Global Plugin Security Options are set to ON or OFF). Everything else appears to work ok, it’s just if I try to go back into the user settings.

I have tried disabling and removing the plugin, but when I install it again the same troublesome settings remain. Is there a way of creating a completely fresh install with none of the old settings so I can start all over again?