I am observing the same behavior Mario is seeing with STunnel 5.54 on Windows Server 2012 R2 running as a client. In my case I am upgrading the client side from STunnel 5.01. The client and server connected without difficulty when both were at STunnel 5.01.

From the STunnel v 5.54 client log:

2019.05.22 16:58:19 LOG7[main]: Found 1 ready file descriptor(s)
2019.05.22 16:58:19 LOG7[main]: FD=332 ifds=r-x ofds=---
2019.05.22 16:58:19 LOG7[main]: FD=352 ifds=r-x ofds=---
2019.05.22 16:58:19 LOG7[main]: Service [EchoClient] accepted (FD=440) from 127.0.0.1:62387
2019.05.22 16:58:19 LOG7[main]: Creating a new thread
2019.05.22 16:58:19 LOG7[main]: New thread created
2019.05.22 16:58:19 LOG7[0]: Service [EchoClient] started
2019.05.22 16:58:19 LOG7[0]: Setting local socket options (FD=440)
2019.05.22 16:58:19 LOG7[0]: Option TCP_NODELAY set on local socket
2019.05.22 16:58:19 LOG5[0]: Service [EchoClient] accepted connection from 127.0.0.1:62387
2019.05.22 16:58:19 LOG6[0]: s_connect: connecting 192.168.128.162:40007
2019.05.22 16:58:19 LOG7[0]: s_connect: s_poll_wait 192.168.128.162:40007: waiting 10 seconds

From the STunnel v 5.01 server log:

2019.05.22 16:58:19 LOG7[5072]: Service [EchoServer] accepted (FD=572) from 192.168.128.104:62388
2019.05.22 16:58:19 LOG7[5072]: Creating a new thread
2019.05.22 16:58:19 LOG7[5072]: New thread created
2019.05.22 16:58:19 LOG7[3724]: Service [EchoServer] started
2019.05.22 16:58:19 LOG5[3724]: Service [EchoServer] accepted connection from 192.168.128.104:62388
2019.05.22 16:58:19 LOG7[3724]: SSL state (accept): before/accept initialization