Session Expired when customizing theme

I’m a newbie to wordpress. I’ve completed a clean install of latest version. When I try to customize the default themes (either twenty 10 & 11), I get the standard log-in screen. I log-in…flashes “successful” but then the log-in screen immediately comes back with error message “Your session has expired. Please log-in again.” I’ve tried most of the suggestions in many of the prior posts on this type of error. I’ve disabled all plugins. I’ve tried with several different browsers including Firefox, Chrome, IE on my PC. On my my iPad I get a different error “too many redirects”. If I cannot get this resolved soon I will have to dump wordpress and find an alternative (no idea what though). Please Help!