An issue does not get synchronized, and it appears on the troubleshooting screen with the category 'Issue must be assigned' and the message 'assignee:Issue must be assigned'.
This error is probably caused because the 'allow unassigned issues' setting in JIRA is set to OFF.
You can fix this problem in two ways:
- Enable unassigned issues in JIRA (this will also allow the target issue to be unassigned)
- In the Backbone configuration, edit the 'assignee' field, and check 'Ignore empty values for this user field'. In this case, syncing from a issue with no assignee to an issue with an assignee will not remove the assignee in the target issue.