Fatal error: Call to a member function on a non-object in /usr/local/apache/php/page.inc on line 68

[Sun Jian]

Hi, occasionally when I tried to access IMP via
http://myserver.com/horde/imp/index.php3, I got this error message:

Fatal error: Call to a member function on a non-object in
/usr/local/apache/php/page.inc on line 68

Refreshing page does not help. But if I just restart my IE, the error
disappears and everything works perfectly.

Does anybody have any idea why this is happening? I am using imp2.2.0 with
php4.

Thanks...

Sun Jian