This is the symptom/issue - I am using a subdomain, and latest Wordpress Rev, as well as PHP on server. When signed in to my site as admin, if I choose admin/customize, I get a new signon screen within a frame and message "Session expired. Please log in again. You will not move away from this page."
I've confirmed this error occurs regardless of theme. And WordPress Address (URL) & Site Address (URL) match.
Last, for what it's worth, I've had this subdomain running for well over a year with no issues, it only shows this symptom recently. As if the Rev of WP were the issue.
UPDATE - I backed up my data, and reinstalled from scratch. The problem is no longer appearing.