We are getting this error when attempting to import the demo content and the whole site is not rendering properly. Additionally, we have tried to import a back-up of a working website from another host but the theme is not registering the ‘save homepage as static page’ & reverts to showing the posts page.
P.S. I should mention we are using multi-site configuration.
The 400 error is usually a server error regarding AJAX in your Dashboard. When you try the import and get the error, right click the page and click “Inspect”. Are there any red errors in the “Console” area?
I managed to work around this by exporting a pre-existing site into new host and raised an escalated ticket with hosting provider re the #400. To fix the homepage display issue I updated the read settings in the back end to display the static homepage (as opposed to using the customiser)