I think SNI only works for https, not imaps/pop3s/smtp.

Looks like the LDAP database is in bad shape, you should restore a backup/dump.

Confirmed on CentOS6 and Ubuntu 18.05 (with last 8.8.15-P20).

Did you upgrade to the NG modules?

If you did, the CLI changed.
https://wiki.zimbra.com/wiki/Zimbra_NG_ ... dmin_Guide

with regard to the RAM memory, the old server has 128 Go while the new one is only 32Go of which 24 allocated to WM. This might be the issue. If Zimbra was installed with 128 GB of RAM available, the installation script tuned both mysql and the Java Virtual Machine for 128 GB of RAM. Now there's on...

Same amount of RAM than previous server?

As Vavai suggested, you should look in /opt/zimbra/log/mailbox.log (and maybe /opt/zimbra/log/zmmailboxd.out) for errors.

How much CPU and RAM on the new VM?

You "cloned" the whole server?

How did you do this?
P2V tool, file copy (rsync, or alike), something else?

Have you read the release notes about enabling TLS 1.3?
https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P20

There was an issue with some kernels, the patch has been patched to solve this.
viewtopic.php?f=13&t=69414