While this is extremely rare you may encounter a web application that loves to open TCP connections to the point where the default pool of available TCP connections are exhausted.

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Services \Tcpip \Parameters]
MaxUserPort = 5000  (Max = 65534)

Just increase the value from the default 5000.

^{Note: You should investigate if the connection behavior is normal for the application as most of the time the connections are only half-open and do not represent legitimate sessions.}