How to debug a TLSv1.3 protocol problem?

[Claus Assmann]

I'm stuck and looking for some hints/help.  I have two MTAs (let's
call them M1 and S8), both built with OpenSSL 1.1.1g. The problem
is M1 cannot establish a TLSv1.3 connection with S8. Using other
MTAs/sites/protocols/tools works just fine, e.g., M1 can send mail
to google using TLSv1.3, and S8 can send mail to M1. Replacing the
server or client with openssl s_client/s_server also works.

I've added some TLS callbacks to S8 which I found in s_cb.c, but
all I get at the end is "SSL_accept:error in TLSv1.3 early data"
(see "full" trace below for the steps leading to this).
Unfortunately I cannot find a way to figure out more details or
what kind of error that is.  Any hints how to determine (and fix?)
the problem?

S8 server side:
info_callback where=0x10, ret=1
info_callback where=0x2001, ret=1
SSL_accept:before SSL initialization
ssl_msg_cb, writep=0, version=0, len=5, ct=256
ssl_msg_cb, before SSL initialization
info_callback where=0x2001, ret=1
SSL_accept:before SSL initialization
ssl_msg_cb, writep=0, version=772, len=512, ct=22
ssl_msg_cb, SSLv3/TLS read client hello
info_callback where=0x2001, ret=1
SSL_accept:SSLv3/TLS read client hello
ssl_msg_cb, writep=1, version=0, len=5, ct=256
ssl_msg_cb, SSLv3/TLS write server hello
ssl_msg_cb, writep=1, version=772, len=88, ct=22
ssl_msg_cb, SSLv3/TLS write server hello
info_callback where=0x2001, ret=1
SSL_accept:SSLv3/TLS write server hello
ssl_msg_cb, writep=1, version=0, len=5, ct=256
ssl_msg_cb, SSLv3/TLS write change cipher spec
ssl_msg_cb, writep=1, version=772, len=1, ct=20
ssl_msg_cb, SSLv3/TLS write change cipher spec
info_callback where=0x2001, ret=1
SSL_accept:SSLv3/TLS write change cipher spec
info_callback where=0x2001, ret=1
SSL_accept:TLSv1.3 early data
info_callback where=0x2002, ret=-1
SSL_accept:error in TLSv1.3 early data

M1 client side:
apps_ssl_info_cb, where=10, ret=1
apps_ssl_info_cb, SSL_connect=before SSL initialization
ssl_msg_cb, writep=1, version=0, len=5, ct=100
ssl_msg_cb, SSLv3/TLS write client hello
ssl_msg_cb, writep=1, version=772, len=512, ct=16
ssl_msg_cb, SSLv3/TLS write client hello
apps_ssl_info_cb, SSL_connect=SSLv3/TLS write client hello
ssl_msg_cb, writep=0, version=0, len=5, ct=100
ssl_msg_cb, SSLv3/TLS write client hello
apps_ssl_info_cb, SSL_connect=SSLv3/TLS write client hello
ssl_msg_cb, writep=0, version=772, len=88, ct=16
ssl_msg_cb, SSLv3/TLS read server hello
apps_ssl_info_cb, SSL_connect=SSLv3/TLS read server hello
ssl_msg_cb, writep=1, version=0, len=5, ct=100
ssl_msg_cb, SSLv3/TLS write change cipher spec
ssl_msg_cb, writep=1, version=772, len=1, ct=14
ssl_msg_cb, SSLv3/TLS write change cipher spec
apps_ssl_info_cb, SSL_connect=SSLv3/TLS write change cipher spec
ssl_msg_cb, writep=1, version=0, len=5, ct=100
ssl_msg_cb, SSLv3/TLS write client hello
ssl_msg_cb, writep=1, version=772, len=512, ct=16
ssl_msg_cb, SSLv3/TLS write client hello
apps_ssl_info_cb, SSL_connect=SSLv3/TLS write client hello
ssl_msg_cb, writep=0, version=0, len=5, ct=100
ssl_msg_cb, SSLv3/TLS write client hello

and here it hangs until timeout.