Seems to have a conflict with user avatar

Hi,
I'm having the same problem..
The entry author is not visible if the author is a Authenticate user, visible if the author is anonymous user.
So, I've enabled picture support (avatar).
It works fine when I use Firefox browser.
Using IE browser I see the messagge: "Fatal error: Cannot use assign-op operators with overloaded objects nor string offsets in /home/mhd-01/mydomain.com/htdocs/includes/common.inc on line 1105"

Thanks