XCOM Server listens on a port to be aware for incoming transfer requests. That port is 8044 by default (and 8045 for SSL-transfers) but can be changed to a different port.
When your XCOM Server becomes unresponsive your Server cannot handle incoming transfers any more.
A possible cause for the XCOM Listener becoming unresponsive is when multiple SSL transfers are received that exceed the "Maximum Clients Range" global parameter value. This will cause an subsequent remotely initiated SSL transfers to fail. In that case please apply fix RO88307.
You can check if XCOM Server is listening by issuíng this DOS-Command:
netstat -ano | find "8044"
This will show a string like this if the Port is responsive:
"TCP 0.0.0.0:8044 0.0.0.0:0 LISTENING 3328"
and no string if not.