Thanks for sharing. In that case I suspect there may be a plugin or combination of plugins resulting in this issue, not necessarily Site Kit considering the same occurs with Site Kit deactivated.
From inspecting your Site Health information I noticed also you have the Health Check & Troubleshooting plugin active. Let’s use this to try and determine the cause. Please follow the steps below:
- Login to your site from a Chrome browser incognito window
- Navigate to “Plugins > Health Check & Troubleshooting > Troubleshoot”. (This will temporarily deactivate other plugins and switch to a default theme – for you only, as a logged in user).
- Check and see if you can create a new post.
- From back in your Health Check plugin click on the “Available Plugins” tab at the top right and then click on the “Enable” option next to one of your other installed plugins.
- Check and see if you can create a new post. If so repeat the process above by activating other plugins one by one, while trying to create a new post each time
- At any stage whereby you’re unable to create a new post take a note of the plugin you’ve just activated.
- You can exit troubleshooting mode afterwards.
If you’re able to locate the source of a conflict you can try and temporarily deactivate that identified plugin and check that plugins support forum.
Let me know how you get on with the above, or if you have any questions.