...
This error is caused if two different versions of the plugin are installed in Confluence.
Please In newer Confluence versions (>= 7.10) this problem happens just by updating the plugin and could be solved by just restarting Confluence. If this does not help then please try to uninstall the plugin and then reinstall the plugin. If this does not help then perform the steps described here: https://confluence.atlassian.com/confkb/how-to-clear-confluence-plugins-cache-297664846.html
...