According to the logs, they are not even trying to reach your site, but these logs only go back a few hours.
It is possible they are doing ghost spam, which occurs when they either cache a copy of your site which doesn’t hit the server at all (and therefore doesn’t work with this plugin).
Your server may have a .gz backup of yesterday’s logs or maybe even the past few days. If you have that, could you please email it to me for review? [email protected]
Those logs would be the same as above but with a larger window of time for me to analyze.