Content body cannot be converted to new editor format

Hi,

I am working on an app with a feature that allows users to insert a macro into the page using the storage format. However, whenever I try inserting the macro using the rest-api ‘/rest/api/content/’, I am seeing a 400 error with “Content body cannot be converted to new editor format”. This feature used to work not too long ago. Does anyone know if there were any changes made to the new editor?

Having the same error when I try to update some pages and will be interested how we can troubleshoot it.
And my test is very simple - I read content and without changes update the page and it fails with the error above (ok i do serialize and serialize) and will try to do it without serialization