Well, the error messages have changed slightly, but at least the clients (Outlook via Activesync w/o KOE, Mail on iOS) do not seem to complain anymore.
Previously the clients would show a popup stating that the username/password was wrong, which I just had to <cancel> multiple times till it went away and synching of Kopano server to the clients resumed.
The new error message:
2020-10-08T06:39:27.011248: [error ] Previous message logged 76 times
2020-10-08T06:39:27.011824: [error ] K-1585: LDAP auth error: Can’t contact LDAP server. Will rebind & retry.
2020-10-08T06:39:27.065428: [error ] K-1582: LDAP search error: Can’t contact LDAP server. Will reconnect and retry.