A mandatory extension policy in the request is not accepted by the server for this resource.
A very odd error. A few days back, it finally took the site down completely. At first I thought it was caused by the Bandwidth Limiting option in Plesk, or the module being active in your Apache configuration. You can fix it by going to the domain in Plesk and choosing for “Bandwidth Limiting”. However, I do not have a lot of traffic going to me site, so instead I looked elsewhere. Turns out it was the google pagespeed service. I disabled everything (although the page is still being served by Google) and the error install went away. I have been watching the site and it seems to be fine again. I’ll give it a week before I try turning parts of the service back on.