Couldn’t save those changes

@meken Sorry, I’ve not seen this particular error before.

I have seen problems with 405 “Method Not Allowed” errors for PUT requests before. That has occurred when the web server configuration, usually managed by a web host provider, has set up rules that only allow some HTTP verbs, and reject PUT. The solution in those cases has been to change the configuration to allow PUT, at least for known legitimate routes like the REST route for this plugin as shown in your error message.

There’s no place in this plugin code that it returns a 409 error. So this does seem like a response that’s coming from some layer of the system outside of this plugin.

Varnish is a cache system, right? I can’t think of a reason that it should be involved in caching anything related to any routes under /wp-json/font-awesome/v1. Any end points under that path would be REST API routes used by this plugin for admin management of the plugin’s configuration. Could you change the Varnish config to just ignore those routes and allow requests to pass through un-varnished?

When you say you visit those URLs, do you mean that you just put those URLs into a web browser?

If so, then what your browser is probably doing is sending a GET request to those routes.

For font-awesome/v1, I would expect it to return 200 because it is valid to send a GET request to that route to list the routes and available methods under that namespace.

I would expect a 404 on font-awesome/v1/config because GET is not a valid method for that route. That route only accepts PUT requests.

Either way, that doesn’t seem to shed too much light on the original problem. It’s possible that your server has different rules for handling PUT requests than GET requests.