Error 18458
From Troubleshooting SQL Server
Troubleshooting
Error 18458
Severity Level 14
Message Text
Login failed. The maximum simultaneous user count of %d licenses for this server has been exceeded. Additional licenses should be obtained and registered through the Licensing application in the Windows NT Control Panel.
Explanation
This error occurs when the server is set for Per Server licensing and the number of attempted client connections exceeds the number of Client Access Licenses for this server.
Action
Obtain additional Client Access Licenses or reduce the number of simultaneous client connection attempts.