Just a guess but Blazor is expecting JSON and you're getting HTML. The HTML is probably a server side error page that is reporting that something failed. Since your server side code isn't translating it to a problem detail or other JSON result then the client side is getting the raw HTML. It isn't checking the status code and so it tries to parse HTML as JSON and fails.
Look at the contents of the HTML page being sent to identify the underlying server error. Then verify that you are not allowing HTML to be returned as the result of errors on the server side. You should be using problem details or something equivalent.
I don't use Blazor myself so I cannot really provide any further instructions on how to handle errors but the docs may provide more useful suggestions.