The <check connection failed> audits can be caused by a system being very busy and having settings that are too small for the CHECKCONNECTION param.
The optimum value for CHECKCONNECTION is dependent on how busy your system is and how long you want a disconnected XAC process to remain running. The the default value of 1000,250 is just a suggestion, it is by no means intended to be the perfect setting for everyone's system. You may want to experiment with values until you come up with one that works in your environment.
If you want to dig deeper and try to figure out why the connections are being dropped, you need to involve the network group and do network sniffs. A network sniff will show who dropped the connection and the nature of the drop. Another thing you could look into with the network group, some routers are configured to detect idle sessions and drop them. This might be the situation at the customer site.
The CHECKCONNECTION option is not needed if you are making a Telnet connection and the Telserv process was started with the ztnT^SENDBRK^ON^DISCONN param set to yes.