An issue does not get synchronized and appears in the troubleshooting section with the category 'User not assignable' and the message, e.g. '"assignee":"User 'ag1' cannot be assigned issues."'.
The 'assignee' part identifies the internal id of the field which is affected.
A user was set to the issue in the sender project and no user with the same name is assignable in the receiver project. At this point of time, backbone only supports mapping user with the same names to each other.
The following steps should be done in order to resolve this problem.