In order to perform its activities successfully, the Internet Information Services (IIS) World Wide Web Publishing Service (W3SVC) must be free of any I/O errors involving file access, memory availability, and network connections. In addition, any scripts that are used must be accessible and running, and any include files must be configured correctly.
Reduce connections or purchase CALs
To resolve this issue, you must either reduce the number of connections or purchase more CALs (Client Access Licenses).
For more information, see the article Error Message: HTTP 403.15 - Forbidden: Client Access Licenses exceeded.