Just a wild speculation, but are your add-on resource files residing under properly namespaced paths?
I faintly recall that we had a similar issue in the past where an add-on of ours used an
i18n.properties file in the top level
src/main/resources directory, which resulted in a collision for some i18n entries with identical names to those from another add-on. (Also hard to reproduce, as depending on add-on installation and/or loading order.)
Moving all our resource files (except for
atlassian-plugin.xml) to namespace subdirectories beneath
src/main/resources fixed that.
PS: The conflict might also have been an identical named template, e.g. using
editNotification.ftl and colliding with the identically named HipChat add-on version (as said, faint memory only)