Summary

Confluence is failing to boot. catalina.out log file indicates that tomcat configuration cannot be loaded:

WARNING [main] org.apache.catalina.startup.Catalina.parseServerXml Unable to load server configuration from [/path/to/confluence/atlassian-confluence-install/conf/server.xml]
	org.xml.sax.SAXParseException; systemId: file:/path/to/confluence/atlassian-confluence-install/conf/server.xml; lineNumber: 92; columnNumber: 31; Error at line [92] column [31]: [null]
		at org.apache.tomcat.util.digester.Digester.createSAXException(Digester.java:1982)
		at org.apache.tomcat.util.digester.Digester.createSAXException(Digester.java:2014)
		at org.apache.tomcat.util.digester.Digester.endElement(Digester.java:1066)
~~~~~~~~~~~~
	Caused by: java.lang.NullPointerException
		at org.apache.catalina.util.ContextName.<init>(ContextName.java:52)
~~~~~~~~~~~~
SEVERE [main] org.apache.catalina.startup.Catalina.start Cannot start server, server instance is not configured

Diagnosis

1. Attempt to start Confluence
2. Open <confluence_install>/log/catalina.out and check last events being logged
3. If you observe Unable to load server configuration from WARNING, then tomcat is failing to load due to configuration error
4. WARNING will also indicate which file is misconfigured, in the example error above, server.xml is incorrect

Cause

Tomcat needs correctly defined configuration in server.xml and other files in order to boot and load Confluence. 

Solution

Go to location in the specified file indicated by line and column values in the error message. Issue might be with incorrect declaration or simply missing character.

You can also use new Confluence installation of the same version for reference for default configuration or content of certain system files.

Example:

Error above indicated failure in Error at line [92] column [31]

Looking at server.xml, we can see that tomcat failed to recognise and load specified context declaration, because it was made incorrectly. Declaration added within its own <Context> ... </Context> tags, as opposed to being added within the existing <Context > ... </Context> tags. Removing or correcting declaration allowed for correct boot.