configuration – Invalid Hostname settings in Drupal 7

I’m running Drupal 7 on Pantheon and have started getting the following error message whenever I try to save content that doesn’t have my personal admin login as author:

You’re attempting to access a Pantheon site via IP address or invalid hostname. Please use a valid hostname.

I’ve found a number of related issues online: Trusted Host settings
How to fix “The provided host name is not valid for this server” in Drupal
Stack Exchange: How to use settings.local.php with Drupal 7?
Stack Exchange: What does “The provided host name is not valid for this server” mean?
Most of these articles reference Drupal 7, but all of the fixes are for Drupal 8.

I’m experiencing this problem in both dev and in production (though user interactions on the live site seem to be fine, thank goodness). It started a week ago, and now I’m functionally locked out from creating new content or editing most of the site. It affects all of my browsers, both of my laptops and occurs whether I’m connecting via wifi or tethering data from my phone (a separate ISP), so this isn’t a local issue.

Any ideas how to resolve this for Drupal 7?