qmail-popup: ERR authorization failed

Attempting to read mail accounts from Outlook Express, it kept asking for the account password.
So this seemed to be an authentication problem.

Dave Sill's The qmail handbook (pag 172) provides the following to verify that checkpassword is working correctly:
/var/qmail/bin/qmail-popup example.com /bin/checkpassword /bin/echo valid pw
I received the following message:
-ERR authorization failed

So there it was, the authentication problem.

Googleing for this problem I've found nothing useful. Some advised to rise the softlimit in the supervise script.

To make a long story short, after looking in all the wrong places, I've finally found the problem was that the "/home" directory and its subdirectories did not have the right permissions.