Is it possible that the browser you are using has been some how configured to prevent caching credentials, I'm pretty sure that this site and probably the webmail addresses for your email providers when viewing in a browser, uses some type of cookie to cache logon credentials (when you check the box that says "remember me" or "keep me signed in" or other words to that effect) in the form of a cookie.
You don't mention which browser you are using or if in fact it is only a problem that you are experiencing inside of a browser, but....
If you have either configured it or them to not save those cached credentials or installed some third party software addin for your browser that is preventing those cookies from being cached (maintained) then it's possible that, that is the problem.
I don't pretend to know or even fully understand how it works but I'm pretty sure that
@Mike could explain it better if he's around.