If (as it appears) add_fsevent is running into a problem during a TM backup, the problem must be on the backup volume. TM wouldn't be adding a FileSystem Event to the volume it's backing up. It is creating files/folders on the backup volume, and each such creation would be an FSEvent.
Try verifying the backup disk. Be prepared to wait a while. You should probably temporarily disable automatic TM backups while doing the verify.
