Issue resolved after removing additional added webmail url from Admin Console > Servers > MTA > Network > Webmail MTA hostnames.

Thanks/Pavan

Intermittent Error while sending email using Zimbra WebMail Client to newly added users, Error Message as shown below: Message not sent; one or more addresses were not accepted. Rejected addresses method: [unknown] msg: Invalid address: OSC <osc@example.com>. com.zimbra.cs.mailbox.MailSender$SafeSen...

Please follow below step to work chat zimlet for Zimbra Chat

# mv /opt/zimbra/lib/ext/openchat/zal.jar /tmp
# cp -rp /opt/zimbra/lib/ext/zimbradrive/zal.jar /opt/zimbra/lib/ext/openchat/zal.jar
# su - zimbra
$ zmmailboxdctl restart

Thank you!

Confirmed that 8.8.15 chat issue resolved with below step which is provided by above thread. Thank you # mv /opt/zimbra/lib/ext/openchat/zal.jar /tmp # cp -rp /opt/zimbra/lib/ext/zimbradrive/zal.jar /opt/zimbra/lib/ext/openchat/zal.jar # su - zimbra $ zmmailboxdctl restart If chat zimlet is disabled...

Same issue with 8.8.15 p1,p2,p3,p4,p5 and p6 as well. showing error for chat when it's login user or admin console. Chat is not working...

Same issue with Chat for ZCS OSE 8.8.15 p6 as well, chat is not working and showing at the user/admin console login.

./install.sh -u
yum-complete-transaction --cleanup-only
yum history redo last
rpm -qa|grep zimbra-openjdk
yum remove zimbra-openjdk

Then try re-installation:
./install.sh