Blazor WebAssembly: The provided ContentType is not supported;
Although this isn't true for the OP, who's getting a "200 Ok" response, this can also happen if your API method returns null (mine was throwing a DB exception and returning null as a fallback).
I had "204 No Content" as the response and "The provided ContentType is not supported; the supported types are 'application/json' and the structured syntax suffix 'application/+json'." as the error at the client end.
I suppose null doesn't look like json :-)
This can happen in the ASP.NET Hosted version of the Web Assembly template project if the Client project is designated as the Startup project. Ensure that the Server project is set as the Startup project.