This error occurs when an import was performed or when a database backup was taken from GXportal. The problem is that there is a site with more backend user licenses assigned than those in the server where the import was made or the backup taken. In this case, additional licenses must be requested, or otherwise the backend role must be removed from one of the defined users.
To determine the site that is causing the licenses problem, access GXportal with GXPADMIN and select the Portals – Licenses menu option.