getting 403 forbidden (guru mediation)

I’m consistently getting

[quote]Error 403 Forbidden

Forbidden
Guru Mediation:

Details: cache-iad2122-IAD 1501102044 1042052109

Varnish cache server[/quote]

when trying to access any page on webpagetest.org on my Ubuntu 16.10 LTS laptop when using Firefox in private mode. Anything I can do to fix?

thx,
frank