Summary

After upgrading Confluence, when reaching it through a browser, the instance is not accessible and displays only a white screen.

Environment

Confluence Server or Data Center, using the Lively Theme plugin.

Diagnosis

Checking the Confluence logs, the following error can be found:

2020-08-06 10:24:52,057 ERROR [http-nio-8080-exec-6] [ContainerBase.[Standalone].[localhost].[/]] log Unhandled exception occurred whilst decorating page
 -- url: /login.action | traceId: 0a542b4626d6f5d9 | userName: anonymous
java.lang.RuntimeException: javax.servlet.ServletException: org.apache.velocity.exception.MethodInvocationException: Invocation of method 'getConfluenceResourceTags' in  class com.atlassian.confluence.setup.velocity.VelocityFriendlyPageBuilderService threw exception com.google.common.util.concurrent.UncheckedExecutionException: java.lang.NullPointerException at /decorators/includes/header.vm[line 96, column 24]

...

Caused by: java.lang.NullPointerException
	at de.scandio.atlassian.applib.conditions.confluence.ThemeIsEnabledCondition.shouldDisplay(ThemeIsEnabledCondition.java:39)

Cause

Due to an issue with the Lively Theme plugin, Confluence pages are not rendering correctly. This issue can happen when this plugin is out of date, making it incompatible with the current Confluence version. 

Solution

With this particular problem, the Confluence administration console will still be accessible so we can update or disable the plugin. To do so, follow the steps below:

1. Access: https://<confluence-URL>/plugins/servlet/upm
2. Filter the apps by User-Installed
3. Search for Lively Theme
4. Click on Update
   
   1. To disable it, click on the app name to expand and click on Disable.

After that, check if Confluence pages are displayed properly again. If necessary, you may reach the vendor support here.