OES 2018 SP1 patched up-to-date 20/Apr/2019.

All of my external LDAP connections to eDirectory are dropped every 60 seconds. For example, on the Dovecot based mail server:

Apr 19 17:43:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 17:44:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 17:45:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 17:53:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 17:54:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 17:55:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 18:13:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 18:14:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 18:15:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting
Apr 19 18:16:10 venus dovecot: auth: Error: LDAP: Connection lost to LDAP server, reconnecting

Notice the gaps between
17:45:10 -> 17.53:10 and
17:55:10 -> 18:13:10

Concurrent Bind Limit: 0 (unlimited)
Idle Timeout: 0 (unlimited)

All connections are over IPv6 LAN subnet. Now going to reproduce over IPv4...

Any ideas?
Thanks in advance.