I have essentially the same issue.  We were successfully sending messages and receiving ACK prior to this glitch.   I am on Ubuntu 12.04 LTS using stunnel 4.50.

appsadmin@MIRTHTEST:/etc/stunnel.d$ sudo tail -f /var/log/stunnel_mirth.log

2013.06.21 17:28:03 LOG7[30980:3077569344]: Service hl7_client prod finished (1 left)

2013.06.21 17:28:03 LOG7[30980:3077569344]: str_stats: 0 block(s), 0 data byte(s), 0 control byte(s)

2013.06.21 17:28:14 LOG6[30980:3073227584]: SSL connected: previous session reused

2013.06.21 17:28:14 LOG7[30980:3073227584]: Socket closed on read

2013.06.21 17:28:14 LOG7[30980:3073227584]: Sending SSL write shutdown

2013.06.21 17:28:14 LOG6[30980:3073227584]: SSL_shutdown successfully sent close_notify

2013.06.21 17:28:24 LOG5[30980:3073227584]: Error detected on socket (read) file descriptor: Broken pipe (32)

2013.06.21 17:28:24 LOG5[30980:3073227584]: Connection reset: 661 bytes sent to SSL, 124 bytes sent to socket

2013.06.21 17:28:24 LOG7[30980:3073227584]: Service hl7_client prod finished (0 left)

2013.06.21 17:28:24 LOG7[30980:3073227584]: str_stats: 0 block(s), 0 data byte(s), 0 control byte(s)

rmcwilli