I know this thread is 9 months old, but to me this issue occurs with “kk Star Ratings” plugin.
I’ve disabled kk Star Ratings and I was able to start the scan and it completed sucessfully.
With a site that has: BulletProof Security, W3TotalCache, qTranslate and other 25 Top of the Line plugins, its awkward that a supposed frontend specific plugin conflicts with P3.
I’ve stopped using kk Start Ratings for now ! ??