Updating some packages (apt update && apt upgrade) break my zimbra server (started to get a "A network service error has occurred." when entering login page)

Upgraded successfully from 8.8.15 to zcs-9.0.0_ZEXTRAS_20240927.UBUNTU20_64.20241001143114

Did all the steps on the documentation, including:

Fix the default globalconfig values by

zmprov modifyConfig zimbraMtaCommandDirectory /opt/zimbra/common/sbin
zmprov modifyConfig zimbraMtaDaemonDirectory /opt/zimbra/common/libexec
zmprov modifyConfig zimbraMtaMailqPath /opt/zimbra/common/sbin/mailq
zmprov modifyConfig zimbraMtaManpageDirectory /opt/zimbra/common/share/man
zmprov modifyConfig zimbraMtaNewaliasesPath /opt/zimbra/common/sbin/newaliases
zmprov modifyConfig zimbraMtaSendmailPath /opt/zimbra/common/sbin/sendmail

Disable DSPAM by

zmprov modifyServer `zmhostname` zimbraAmavisDSPAMEnabled FALSE
zmlocalconfig -e amavis_dspam_enabled=false
zmamavisdctl restart

Restarted zimbra services, checked the server, everything was fine.

So until this point I had a functional ZCS 9 from the 20240927 build.

Ran apt update, found the folloiwing packages available to be updated:

zimbra-mta-patch/unknown 9.0.0.1629795032.p18-1.u20 amd64 [upgradable from: 8.8.15.1723777774.p46-1.u20]
zimbra-patch/unknown 9.0.0.1629795032.p18-1.u20 amd64 [upgradable from: 8.8.15.1733155846.p47-1.u20]
zimbra-proxy-patch/unknown 9.0.0.1629795032.p18-1.u20 amd64 [upgradable from: 8.8.15.1701429424.p45-1.u20]

Updated these packages, restarted zimbra services, as documented. Checked services again, all services running. Checked zimbra.service with systemctl status zimbra.service....all green...

And now, when attempting to login to my mailbox I now get the "A network service error has occurred." Red error message.

Checked my mailbox.log, found:

[jetty-util-9.4.46.v20220331.jar:9.4.46.v20220331]
at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: com.zimbra.common.service.ServiceException: error while proxying request to target server: Service Unavailable
at com.zimbra.common.service.ServiceException.PROXY_ERROR(ServiceException.java:409) ~[zimbracommon.jar:9.0.0_ZEXTRAS_20240927]

Reverted my snapshot prior to the update.

If someone could help me diagnose this, that would be terrific.