This function returns log by id.
jetapi backup -F getLog -D '_id=5b5f8e5ed5ac5e09aa5d3ee2'
https://hostname.example.com:2087/cpsess##########/cgi/addons/jetbackup/api.cgi?function=getLog&_id=5b5f8e5ed5ac5e09aa5d3ee2