500 error on Add New Menu Item and changing Global Config

[slselwyn]

As soon as I try to Add New Menu Item the following error appears: "Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error."
It also happens if I try to save items in Global Configuration. I can Add New Menu and many other things are working normally.
This is on a Windows 2008 server with Plesk and Joomla was installed through Plesk.
We have tried temporarily on another site changing all the permissions to Everyone full control to see if it might be a permissions issue but it made no difference.
Any suggestions would be really appreciated.

[vivec]

Make sure your 'configuration.php' is writable and (then) turn of all 'SEF' options !

[humvee]

Please run the Forum Post Assistant to provide the information to help us try and resolve the issue.
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