Connection table full complains in messenger logs.

Your Messenger runs fine over several hours or days and then users cannot login anymore. Messenger log shows info like example bellow:

11:42:08 700 SPL Request (Get Other Status) [0xB103]: GFL - Requested Userid not found in directory
11:42:08 700 SPL Request (Get Other Status) [0xB106]: GFL - Host is unknown to system
11:42:08 700 SPL Request (Get Other Status) [0xB106]: GFL - Host is unknown to system
11:42:23 700 SPL Request (Get Other Status) [0xB106]: JBN - Host is unknown to system
11:42:25 700 TCP ** NEW PHYSICAL CONNECTION REQUEST REJECTED, TABLE FULL **
11:42:26 700 TCP ** NEW PHYSICAL CONNECTION REQUEST REJECTED, TABLE FULL **

Go to the Messaging agent's Agent Settings Tab in GW Admin.  Under Performance Preferences at the bottom of the page, see what Maximum number of users: is set to.  It might be set to 100. 

That value should default to 5120 and be able to set as high as 100,000.  You can set to the max of 5000 (if more is required you can use the switch /maxusers=XXX in the startup files).  Restart agent and that shall fix it.