Symptoms

When accessing the Workflow designer for any workflow (Diagram mode) you will only see a blank page like in the image below:

Cause

1. Prior to JIRA 6.1: The Adobe Flash Player is not updated to the latest release. Some browser's like safari do warn the user that the Flash Player must be upgraded, but some other browsers (like Firefox) do not throw any warning regarding to this problem.
2. In JIRA 7 onwards: Tempo Timesheets plugin is installed and some of the core plugins require updating: https://tempoplugin.jira.com/browse/TT-3020 

Resolution

1. Upgrade your Flash player version to the latest release. This will make it work in all browsers.
2. Please temporarily disable the Tempo plugins to confirm this is the cause.  Update all Tempo plugins and dependencies to the latest supported versions and contact Tempo support.