I am running ArcGIS for desktop 10.2.1 on Windows 8 and I am having problems returning concurrent use licenses. The licenses were borrowed while I was at another workplace. I have started a new job and would like to use the licenses on their server. Unfortunately I can't return the licenses as I no longer have access to the previous license server - every time I try return the license I receive an "Error Returning License" message. I have also tried uninstalling ArcGIS, however, when I reinstall ArcGIS the licenses are already borrowed when I open the ArcGIS Administrator. I am not sure where the license data is stored. I assume in the registry.