This critical update fixes a compatibility issue with CGP 5.3.1 which was released yesterday.
If you are going to install CGP 5.3.1 please make sure all Communicators have been upgraded to this release (1.3.13), older releases will cause excessive session consumption which will eventually make a CGP server run out of resources.
The new release can be downloaded from the usual location at http://itoolabs.com/communicator/download/