I have the same problem, in these days a lot of 403 errors…
While I’m surfing my site, or while I’m managing it from the BackEnd (editing or updating articles, updating plugins, ecc) somethimes this Error 403 happens, so I can’t continue access the site; after a 1-2 minutes, if I try again or refresh the page, I can see the content as usual.
You can see the sudden increase of 403 errors from this chart in Search Console:
https://s3.amazonaws.com/diigo/310428_331172498_24447484?AWSAccessKeyId=0R7FMW7AXRVCYMAPTPR2&Expires=1474795679&Signature=ZEEwLV9fx1Zqr6jtABuhaibBXGc%3D
I dont’ know how to manage the situation. I would be sorry to be obliged to change plugin, I like IThemes Security! ;(
