I’ve checked two sites created old days on Stackbit (currently Netlify Create) and both has problem with new post/page creation. There is error displayed in Netlify Create panel and no more information. Same when trying to duplicate some post. Looks like issue appears last week so maybe there was some release of new version of Netlify Create and that’s causing issue? There is no problem to add page locally in repo and push changes. Problem is in Netlify Create panel. I’ve checked other site on other account and there is same issue so I believe that could be some generic issue with Netlify Create.
I’ve created totally new account and page with some template so I can see now I’m able to create page there where still issue is on existing account on pages created some time ago.
What I see more, even changes pushed through repo do not updates content in Panel until restart of container.
Even then I see changes but when clicking Publish button, I see no changes so I’m unable to publish through Panel.
My page is https://mokrabka-03abe.stackbit.app/
Hi @TomBonior 
Thank you for reaching out and apologies for the delay in response.
I’ve confirmed with our Create developers that there was an update on our end. It appears your projects were created pre-CSI and because you are on the free tier we stopped supporting them and have encouraged users to migrate to CSI instead. Pre-CSI means that you are using a depreciated format, such as a stackbit.yaml file: stackbit.yaml [Deprecated] | Netlify Create Docs
CSI has replaced the previous approach. This is a public repo that anyone can use in Create - GitHub - stackbit-themes/nextjs-markdown-starter
You can duplicate this to your projects using create.netlify.com/import and choose “Duplicate”.
If you have any further questions, encounter additional issues, or need assistance with anything else, please do not hesitate to reach out.