I started seeing this when I moved to 4.8.x for no apparent reason. However, it has a fairly easy fix -- create a static validationKey under the machineKey section in your web.config. It is generally set to validationKey="AutoGenerate,IsolateApps" however for some reason that will fail from time to time.
The following resources will talk about that: @Microsoft
Key generator @DevelopmentNow
I haven't done this for my own site yet, but I have done this for some client sites and it eliminated their issues. As a note, I believe the AutoGenerate,IsolateApps was put in the default web.config because there is a known issue (see SecurityBulletin) in the v13wstat3 stuff and a known value.
Good luck!