Got the same problem, with the Viewer 2, up to date :matte-motes-not-even:
I added slplugin to exceptions of my AV. I also added slplugin.exe in my firewall (as SecondLife.exe & other SL exes)
I turned compatibility mode of slplugin.exe to Win 95.
I ran SL as admin, & not.
Nothing changes. Still got the media_webkit error