You could try to remove the '.html' extension from 'signInPage' in your SecurityConfig class like this:

.formLogin(
    form ->
        form.loginPage("/signInPage")
            .permitAll()

The .html isn't necessary in your request as Spring (Spring mvc, security and thymeleaf) will route it to the appropriate view for you.

Also, do you have a @Controller class which returns your Sign-in page when requesting /signInPage?

For example, a very minimalistic code snippet:

@Controller
public class PageController {
 @GetMapping("signInPage")
 public String signInPage() {
  return "signInPage"; // this will lookup the signInPage.html in your resource/template directory
 }
}

Btw, I'm new to stackoverflow so please let me know if something is not according to the guidelines