If you set the regional setting of a site collection to Turkish, you get the “Operation is not valid due to the current state of the object” error when you try to change the permissions of a list or a library. This is a bug and there is a non-public hotfix for this bug. You can call your local Microsoft Support phone number and ask for the hotfix for the article number KB931636. After you apply the hotfix, you have to run the "SharePoint Products and Technologies Configuration Wizard" in order to make the needed modifications to existing installation.

There are two reminders for you about this issue:

1. This hotfix can not be uninstalled.
2. The hotfix will be included in Service Pack 1 of WSS 3.0 and MOSS 2007.