Zimbra Forums

Which document you are following ? If you follow release notes for Patch 40, there it is clearly mentioned about deprecated encryptions https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P40 With JDK 17, weaker Kerberos encryption types like 3DES and RC4 have now been disabled by default. This can ...

There is no any open vulnerability found or reported that allows unauthenticated user to read mail.
You can follow Zimbra's Security Center page on this wiki, you will find all reported and fixed vulnerabilities.
https://wiki.zimbra.com/wiki/Security_Center

For 10.0.8 To enhance security, when using External LDAP for authentication, the behaviour of authentication of admin accounts has been changed. Earlier, the admin accounts could be authenticated locally though zimbraAuthFallbackToLocal set to FALSE. The behaviour has now been corrected for admin a...

I hope you are following this guide https://wiki.zimbra.com/wiki/LDAP_Authentication
check troubleshooting section as well.
and see what error you are getting in mailbox.log related to authentication -- like 525 - user not found, or 52e - invalid credentials, etc

check all services are running on this single server instance.
check 389 port is open to connect with LDAP

Please refer this post for generating new self signed cert
viewtopic.php?p=310667#p310667

go for Ubuntu 20
https://zimbra.github.io/documentation/ ... quirements

check in mailbox logs for these specific users to find the reason of not getting connected

latest patch for 8.8.15 was P45 and not P23.
https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P45

8.8.15 had EOL last December, you will have to consider to upgrade to Daffodil (10.0.0) version.

Zimbra team recommends you to go for Daffodil version (10.0.0).
9.0.0 is near to EOL.

but if you still want to upgrade to 9.0 as intermediate solution then here is the document to follow
https://zimbra.github.io/zimbra-9/upgrade.html