Todays upcp, caused issues with mails

one of my server got daily update a couple of hours back,

during the update cPanel was throwing 500 "Internal Server Error"
but after it completed, WHM and cPanel started working but I still have issues with mails

All mails reaching destination mailbox and get downloaded over pop3, but I since update
none of our hosted account is getting mails or notification if they are configured on IMAP.

IMAP service appears UP under service status, Restarting IMAP or pop3 didn't solved the issue.

Is Anyone else having this issue ?

 

I do see my mailbox getting connected and then disconnect after checking/retrieving mails via IMAP

But I found a strange issue.

Earlier all my IMAP clients were connected using PORT 143 and "Security Type" STARTTLS (Accept All Cert....) in Android GMAIL APP.
this I am talking about more than 10 android which were working just fine till this mornings UPCP

now many of those phones are not getting new mails on phones via IMAP, however if the user access the mailbox on his PC via Outlook, the mails do get downloaded, and android didn't synced those until.
You go back to Gmail App Setting on phone and select "NONE" in Security Type under incoming server setting.

However I did tail on the maillog and found one thing preventing the IMAP Sync

Code:

Apr 12 16:59:19 mpi dovecot: imap-login: Error: SSL: Stacked error: error:14094416:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate unknown: SSL alert number 46

Apr 12 16:59:19 mpi dovecot: imap-login: Disconnected (no auth attempts in 2 secs): user=<>, rip=1xx.1xx.85.11, lip=67.xxx.xxx.xx, TLS handshaking: SSL_accept() failed: Unknown error, session=<vhBd708wAq56sVUL>

Nulled my & Server IP above

SSL on Hostname is properly installed and working fine for whm/cpanel/webmail.

If I am wrong, why only after today's cPanel this problem started, and lastly If i configured the mailbox wrongly on Android App
why this was working till earlier today ?