PAM authentication failure with HTTP Basic auth and mod_auth_pam on Apache

I'm trying to set up HTTP Basic auth with PAM on Apache (running on Ubuntu 10.04).

I have a VirtualHost setup with SSL and the options below for HTTP Basic authentication:

AuthBasicAuthoritative off
AuthPAM_Enabled On
AuthType Basic
AuthName "PAM"
require valid-user

However, PAM refuses to log me in, despite having the right credentials.

Here's what I have in auth.log:

unix_chkpwd[25522]: password check failed for user (test)
apache2: pam_unix(apache2:auth): authentication failure; logname= uid=33 euid=33 tty= ruser= rhost=XX.XX.XX.XX user=test

Could this be related to HTTPS sending the encrypted password to Apache, and Apache failing to decrypt it somehow, instead sending it directly to PAM?

Help is greatly appreciated.