I am trying to: edit an existing layout and upon save it keeps showing There was an error during the ajax request, make sure the data you send are in json format.
This could be the server blocking AJAX requests.
The fastest way to find out if this is the case, is to grab a copy of the site with the Duplicator Plugins and deploying it on a MAMP or WAMP local instance.
If then the issue persist, please try disabling all plugins but Toolset and using a native WordPress theme.
I could not replicate such issue locally, but I saw it a few times in the Forum and it was usually due to the Server blocking AJAX calls.
It turned out to be a conflict with Breeze plugin. I deactivate Breeze, updated the Layout I was needing to work on and saved. Then reactivated Breeze.