libapache2-mod-auth-pam: Cannot open password file

After upgrading to Ubuntu Feisty, pam-based password authentication for Apache (2.2) didn’t work anymore. In the logs I found only some very useless entries like

[Wed Apr 25 20:36:20 2007] [error] Internal error: pcfg_openfile()
    called with NULL filename
[Wed Apr 25 20:36:20 2007] [error] [client xxx.xxx.xxx.xxx] (9)
    Bad file descriptor: Could not open password file: (null)

After searching the web for a while I finally found a solution which works perfectly for me: Just disable all the other authentication modules using the following directive:

AuthBasicAuthoritative off