I was receiving this “Invalid AJAX response” issue along with some SSL certification authentication ones (whose messages I unfortunately cannot reproduce). I tried the Remote Fetch Method tweak above with no luck, but, given the SSL issues, I tried setting the Remote Verify Peer (SSL) option to No which worked and allowed me to update to the latest version of Grav.
I actually went back and set it to the default of Yes and the error message never came back. Seems like a bug maybe was introduced to the server new Grav updates resides on that older versions don’t like.
Anyway, I hope this tip might help someone out there. Would like hear from someone from the Grav team perhaps help us understand the issue.
