N360 Backup Error

Recently been getting red cross and "Error" when I do a backup. I am aware the N360 will only backup user files such as documents, music, photos - it will not accept system files. I looked at the list of files in the backup and I couldn't see any files that N360 could object to. I have used N360 for several years without significant issues. Has anyone experienced this backup error problem. Incidentally I did a test - I added a new document file and despite saying backup error the file appeared on the list of backed up files. And I have already uninstalled and reinstalled N360 but it continues to have this backup error issue. Also where are the N360 error logs located ?