If you have applied KB883543 onto your Exchange 2003 box, you will find that any rule you try to create in OWA that tries to forward to another user will now bring up the dialog box.
This rule cannot be saved. Wait a few minutes and try again. If the problem persists, contact your administrator.
The only option at the moment is to uninstall this hotfix which restore functionality...
I don't believe, at time of posting, there is a fix for this hotfix..