I am getting a "response is not a valid json response" error when trying to save Block widgets. I have tried everything I can think of; reset permalinks, deactivated all plugins, reset to a default theme (TwentyTwenty-One), and deleted all widgets, but get the same
{"code":"rest_no_route","message":"No route was found matching the URL and request method.","data":{"status":404}}
error when checking in Inspect. Tried in Edge and Chrome. Using Ubuntu 20.04 and PHP 7.4.3 with WP 6.0, although it was happening in previous WP versions. Nothing unusual in wp-config.php, and nothing in sites-enabled that I can see. I do have multiple wordpress instances (3 total) running on the server. Saving a post in the block editor works fine, it's only in trying to save widgets. It's either something in the database(s), or something in Nginx, probably, but I'm hitting a wall. Any suggestions?