After recently installing a plugin either by placing the jar file in
<CONFLUENCE_INSTALL>/confluence/WEB-INF/lib directory or via the Administration > Plugins page, confluence fails to start up with the following in the atlassian-confluence.log file:
A similar issue was discovered at CONF-14126, however this is not considered a bug as plugins in Confluence should not be installed via the classpath (i.e. put in the
<CONFLUENCE_INSTALL>/confluence/WEB-INF/lib directory). They should be only installed via the
Administration >> Plugins page within the Confluence Administration.
When confluence starts up it loads the classpath plugin first and then discovers that there is a newer version of the plugin in the database. In order to install the new version it first needs to uninstall the old one, but because it is a 'classpath' plugin it cannot be uninstalled hence the exception.