Hi Maria,
I have checked your site mentioned in the previous thread and found that it is having some authentication problem and logging out on every click or a few time spans. The error you are facing might be due to the mod_security being enabled on your site, and that causes the issue in calling the Cornerstone endpoints while saving the content. If that is enabled, please disable it and check if that resolves your problem or not. I would suggest you go through the following article, which may help you with this.
https://www.interserver.net/tips/kb/how-to-disable-mod_security-and-why-it-is-not-recommended/
If you don’t have access to do that, please get in touch with your Hosting Service provider to do that.
Hope it helps.
Thanks