Upgrading the Support Tools Plugin throws a java.lang.AbstractMethodError, which prevents pages from being rendered. This effectively renders Confluence useless.
The following error is thrown in the logs:
And this error shows up in the UI:
Steps to Reproduce
Launch Confluence v.5.2.3
Upload the plugin file for STP v.3.5.20
Navigate to the dashboard
Confluence versions 5.2.3, 5.2.5