Error: Apache2 + mod_auth_mysql - Internal error: pcfg_openfile() called with NULL filename

You use Apache2 and mod_auth_mysql to password-protect web folders. In Apache's error log you see these errors:

[Wed Jun 11 17:02:45 2008] [error] Internal error: pcfg_openfile() called with NULL filename 
[Wed Jun 11 17:02:45 2008] [error] [client 127.0.0.1] (9)Bad file descriptor: Could not open password file: (null)

Solution

Add

AuthBasicAuthoritative Off
AuthUserFile /dev/null

to your mod_auth_mysql configuration and restart Apache.