In some template, the icon resoures are referenced using the Plugin Download Resource; with an URL like <host>/<contextPath>/download/resources/<pluginCompleteKey>:<anyModuleKey>/icon/myIcon.gif

In the current case, we had <host>/<contextPath>/download/resources/fr.alkaes.minyaa.jira-plugin-minyaa-core:Actions/icon/myIcon.gif , where "Actions" is moduleKey that does not exist anymore.

In cause no issue for JIRA 7.x, in JIRA 6.4.x, the rendering fails.

Resolution : Use an URL without module key like <host>/<contextPath>/download/resources/<pluginCompleteKey>/icon/myIcon.gif