This message indicates that Exalate couldn’t update the issue. This might be due permission problems but more often is about the issue being in a status (like closed) that doesn’t allow to update the issue (only add comments). From the incoming script, you can avoid this error by doing:
//Incoming sync, place this on the top of your script
if(!firstSync && nodeHelper.isIssueEditable(issue)) {
issue.comments = commentHelper.mergeComments(issue,replica)
return;
}