- Made sure session.cache_limiter is set to "none" as suggested by Moshe
  and Ax.

.htaccess

@@ -30,6 +30,7 @@ ErrorDocument 500 /error.php
    php_value session.cookie_lifetime 2000000
    php_value session.auto_start      0
    php_value session.save_handler    user 
+   php_value session.cache_limiter   none
    php_value allow_call_time_pass_reference  Off
 </IfModule>
 

includes/conf.php

@@ -20,17 +20,9 @@
 #
 # PHP settings:
 #
-#   The following PHP settings are known to work well:
-#     magic_quotes_gpc                0
-#     magic_quotes_runtime            0
-#     magic_quotes_sybase             0
-#     track_vars                      1
-#     short_open_tag                  1
-#     register_globals                1
-
-# Avoid "page has expired" problems when browsing from your cache or
-# history after having filled out a form:
-// ini_set("session.cache_limiter", "");
+#   To see what PHP settings are known to work well, take a look at
+#   the .htacesss file in Drupal's root directory.  If you get 
+#   unexecpted warnings or errors, double-check your PHP settings.
 
 # If required, update PHP's include path to include your PEAR directory:
 // ini_set("include_path", ".:/path/to/pear");