You can enable tracing of the HACL by adding the following to the Session Properties resource attribute:
SESSION_TRACE ECLSession=3 ECLPS=3 ECLCommEvent=3 ECLErr=3 DataStream=3 Transport=3 ECLPSEvent=3
The trace parameters should be listed without any new line characters. It is acceptable if the parameters wrap in the text box.
The Telnet/TN3270 server should have logs that may help as well.