Any idea why preview would stop working?

Live preview?

like when editing an channel entry? (just to confirm)

yes, I can edit, but the preview screen shows the error.

I got that when testing an msm thing where I had the wrong site in the index file.

any chance it’s msm?

No, very simple site.

Welp- weird one! @Bryan figured it out. Basically, apache sometimes strips out some header info before it hands it over to php. And in this case, we’re passing a token in that auth header. Which is why it failed.

Good news- adding this to your htaccess file said ‘please don’t do that’ and now it works:

SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1

And we’re going to get that documented and some more informative error messaging for when that happens.

Geeze! Thank you for figuring it out! My hero!

I was having the same problem. Just wanted to say thanks for posting the solution.

Same here, and that fixed it. Many thanks.

WOW! I stumbled on this too! I figured it was something with that token. Lifesaver.

I encountered another problem with live preview.

• You can only invoke Live Preview from domains that are configured as Base URL, CP URL, Site URL or added as allowed_preview_domains config override.

Well with this new server implementation the problem was narrowed down to an Apache setting. In my case, it was due to a misconfiguration of our Cloudflare SSL/TLS settings used for the proxy. the HTTP_X_FORWARDED_PROTO was coming back as http. That alone would prevent the preview from working. Changing the TLS/SSL settings to full or full (strict) got that value to be https and preview worked. Special pat on the back goes to EE’s Bryan for identifying this.

Just had this same issue crop up on an established site, the htaccess line fixed it, thanks! Is there a reason it became an issue for so many people all of a sudden? Did something change in an EE upgrade?