It's a bug in the new Add-On implementation that occurs with pages with big memory footprints, often due to embedded images (which is the default for a ipynb noetbook) https://github.com/readthedocs/readthedocs.org/issues/11693

Suggested workarounds from the ticket:

Temporarily, if you are hitting this error you can either disable Addons for your project or try to reduce the response size for the page. If you are using a notebook, you could try to output image as files instead of using inline data in the HTML for the images.