Backbone stores its data in the JIRA database and the
JIRA-Home/data/backbone-issue-sync directory. Additionally, Backbone stores some temporary information in the "java.io.tmpdir". You can look up the real path for the "java.io.tmpdir" in the global JIRA administration (System Info).
Please make sure that the OS user who started JIRA has read and write access to the above mentioned directories.