Symptoms

There are several ways this error could occur, from a theme failing to individual images not appear on a page.  Errors with stack traces including the following line will be found in the logs.

Caused by: java.lang.NoClassDefFoundError: Could not initialize class sun.awt.X11GraphicsEnvironment 

Diagnosis

Run a tail of the atlassian-confluence.log file and reproduce the situation you are facing.  If you see the above line in the resulting stack trace then you are affected by this issue

Cause

Confluence is running on a headless server but the JVM is not set as running on a headless server

Resolution

 These changes require a restart of Confluence to take affect

Add the following argument to your java start options for Confluence, if you are not sure how to change these settings please have a look at our Configuring System Properties documentation

-Djava.awt.headless=true