You have enabled the domain tool which is a function to disallow users to add new domains that were not aprooved by the admin.
To disable the domain tool, go to system > interface config and disable the checkbox for the domain tool.
i have tried to reinstall the interface and also extract it from the SVN to see if this would help
I hope that you have not copied the svn code to oyur system as the svn version is not compatible with the current stable database. So using the svn code will break your setup.