Some more testing:
- If I make a new clone (Duplicate Post plugin), it works fine to edit with Cornerstone (draft and pulblished)
- If I change language on page (Settings, changing from Swedish to English - Polylang plugin, Swedish is default language), it will no longer be editable in Cornerstone: "The preview could not load due to the iframe response being incomplete. This is most often related to a plugin conflict, or customizations introducing a PHP error."
- If I change the language back again (from English to Swedish), it works again.