Added Integration Service to TSIM server using TSPS console and it remained connected to TSIM for few days after that IS got disconnected and not able to connect again.

TrueSight.log showed, TSIM lost connection with Integration service due to socket failure and after that it continuously attempted to connect to IS but IS refused the connection requests from Agent Controller.
 

INFO 10/30 11:31:54 pronet_cntl [AgentConnector-10003] 101574 Close the connection to agent 10003 
INFO 10/30 11:31:54 pronet_cntl [AgentConnector-10003] 101564 The status of the agent in DB 10003 will be changed from 102 to 103 
INFO 10/30 11:31:54 pronet_cntl [AgentConnector-10003] 101565 10003 disconnected. 
ERROR 10/30 11:31:54 pronet_cntl [EventWorkerThread_Event# ] Failed to send a Reply message for message id: <Server-ID:0-1540920714594> Missing Resource String 
WARN 10/30 11:32:06 pronet_cntl [AgentConnector-10003] 101805 Agents (AgentId:10003 AgentName:tsis.xxx.com AgentIP:xx.xx.xx.xx) socket closed. 
WARN 10/30 11:34:32 pronet_cntl [Thread-270] 101795 Agent (AgentId:10003 AgentName:tsis.xxx.com AgentIP:xx.xx.xx.xx) is unreachable. Will contact the agent in next poll. 
INFO 10/30 11:34:32 pronet_cntl [Thread-270] 600002 Closing the connection to agent with ID 10003 
INFO 10/30 11:34:32 pronet_cntl [Thread-270] 101574 Close the connection to agent 10003 
INFO 10/30 11:34:32 pronet_cntl [Thread-270] 600002 Re-establishing connection to agent with ID 10003 
INFO 10/30 11:34:32 pronet_cntl [AgentConnector-10003] 101554 Connecting to TCP agent 10003 ip( tsis.xxx.com ) port(12124)... 
WARN 10/30 11:34:32 pronet_cntl [AgentConnector-10003] 101756 AgentConnector: Connection refused (Connection refused) to Agent (AgentId:10003 AgentName:tsis.xxx.com AgentIP:xx.xx.xx.xx)

TrueSightAgent_<IntegrationServiceHost>.log showed below error when Agent controller attempted to connect to this IS.
 
INFO 10/30 11:18:54 Agent [AC_Listener] 102456 Agent received a connection request from /xx.xx.xx.123 
WARN 10/30 11:18:54 Agent [AC_Listener] 102736 Agent denied the connection attempt from /xx.xx.xx.123 as it is already connected to another server ( /xx.xx.xx.456 ). 

Later, It was identified that another TSIM server (xx.xx.xx.456) also had this IS (tsis.xxx.com) added to it previously and the server was brought down purposefully but the IS entry was not removed.

But recently the TSIM server (xx.xx.xx.456) was started again and TSIM application also got started due to automatic startup setting.

This TSIM server then attempted to connect to IS and during that period IS had lost connection with it’s current TSIM server (xx.xx.xx.123) so IS accepted connection request from (xx.xx.xx.456 ).and started refusing connection requests from  (xx.xx.xx.123).

Since IS can connect to only one TSIM server at any point of time, disconnected and removed the integration service from TSIM (xx.xx.xx.456) and after that TSIM server (xx.xx.xx.123) could connect to IS again.

In one of the scenario, after OS patching activity, same errors observed in the logs as mentioned above, and ISN stops to connect back to the TSIM, Identified the server was not rebooted properly and hence the ISN failed to connect back, Reboot the server once and restart the ISN service, it should help if there is no connectivity issue.