This can also be the result of using Windows Server 2003 Web Edition, which is hard coded to only allow 10 concurrent connections (under the theory that a web server doesn't need to support multiple Windows Networking clients). If this is the case, your only option is to not use Windows Server 2003 Web Edition.
Finally, if you are using "Per Server" licensing and you have only added 10 licenses, this would also prevent additional connection attempts. In this case, you need to purchase and add additional licenses using the "Licensing" administrative tool. In addition to having the appropriate number of licenses, I also generally recommend using "Per Seat" licensing to prevent these kinds of licensing blocks from being enforced.
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This was first published in February 2006