I had to refresh my memory to see what HTTP Error 401 - Unauthorized was exactly...
So bascially you can get this error if you are not authorized (no subscription) or failed to authenticate (wrong username/password). Plus some other corner cases.
Perhaps something like:
"We are sorry but we are not able to authenticate you. You have to subscribe to access these pages. If you are already subscribed, check you gave proper credential in the login step."
Just in proper English, of course (I am French), perhaps with more details (what to do if you forget your password, etc.). And a link to the public part of the site, and to the login page.