Validation server starts and runs fine for a day or more, then suddenly will stop authenticating users.  

ValidationEngine 0 18484 "text=Access DENIED Provider [VSAuthWebServiceClient] Thread pool exhausted, please increase the size and re-start the service. [18484]

This is resolved in the 9.8 Enterprise Gateway.  

In 9.7 and earlier, the thread pool is constant, so it is possible to hit the thread pool upper limit.  In 9.8, there is a dynamic addition of resources, so the thread pool upper limit should not be reached. Upgrading to 9.8 or later is recommended.