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I tried below method to resolve the problem.

Check point 1. Case Sensitive collation in your MySQL

If you are using MySQL as database, you should make sure if your database is created as below command. If not, you should backup your confluence, drop and create your database.

CREATE DATABASE confluence CHARACTER SET utf8 COLLATE utf8_bin;

I thought my database is created as utf8_bin, so may have no issue, but actually you will face lots of issues very soon.

Check point 2. Apache proxy

If you are using Apache proxy, you should check if you change the setting like below. Note that you should put your own domain name for proxy server or just 127.0.0.1 is good enough if that is installed internally.

<Server port="8000" shutdown="SHUTDOWN" debug="0">
    <Service name="Tomcat-Standalone">
        <Connector port="8090" connectionTimeout="20000" redirectPort="8443"
                   maxThreads="48" minSpareThreads="10"
                   enableLookups="false" acceptCount="10" debug="0" URIEncoding="UTF-8"
                   protocol="org.apache.coyote.http11.Http11NioProtocol"
                   scheme="http" proxyName="127.0.0.1" proxyPort="80"/>

        <Engine name="Standalone" defaultHost="localhost" debug="0">
            <Host name="localhost" debug="0" appBase="webapps" unpackWARs="true" autoDeploy="false" startStopThreads="4">
                <Context path="" docBase="../confluence" debug="0" reloadable="false" useHttpOnly="true">
                    <!-- Logging configuration for Confluence is specified in confluence/WEB-INF/classes/log4j.properties -->
                    <Manager pathname=""/>
                    <Valve className="org.apache.catalina.valves.StuckThreadDetectionValve" threshold="60"/>
                </Context>

                <Context path="${confluence.context.path}/synchrony-proxy" docBase="../synchrony-proxy" debug="0"
                         reloadable="false" useHttpOnly="true">
                    <Valve className="org.apache.catalina.valves.StuckThreadDetectionValve" threshold="60"/>
                </Context>
            </Host>
        </Engine>
    </Service>
</Server>

Check point

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3. Checking router settings if you forward Synchrony port

Based on the log on my router, I got lots of DDoS attack from external servers, and it does not necessary to open that port. Technically Confluence already has a proxy feature to support as below - the action you need is to delete that port(default:8091) in the forwarding list.

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Reference: https://confluence.atlassian.com/doc/possible-confluence-and-synchrony-configurations-958779064.html

Check point

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4. Modifying /etc/security/limits.conf in my CentOS 7 server.

* hard nofile 500000
* soft nofile 500000
root hard nofile 500000
root soft nofile 500000

Check point

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5. Increasing max memory by modifying setenv.sh

One of the possibility is caused by memory limits. In my case, mx used the default value (1024m), so I changed its size to 2048m. 

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Once your modification is done, you should restart your server to take effect.

Check point

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6. Adding more memory to Synchrony

The default memory size for Synchrony is 1g, and below shows the memory is assigned by 2g. That should be added at the end of <confluence home directory>/bin/setenv.sh

CATALINA_OPTS="-Dsynchrony.memory.max=2g ${CATALINA_OPTS}"

Check point

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7. Modifying a value of maxThreads.Connector in <confluence_home_directory>/conf/server.xml

Actually this approach is not recommended, because it could cause poor performance delay in GC(Garbage Collection) sometimes.

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Once your modification is done, you should restart your server to take effect.

Check point 7. Change port number for collaborative editing

Other approaches

https://confluence.atlassian.com/doc/troubleshooting-collaborative-editing-858772087.html

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