AWS Cognito Angular SpringBoot Oauth2 - invalid_token error

we are using AWS Cognito for Oauth2. Our UI is built on Angular. After my user logs in, I initiate a call to Cognito to get Authorization token. I am using Authorization Code Grant with PKCE for getting the token from Cognito. After getting the toke from Cognito, I call my Spring Boot REST service. When calling Spring Boot service from Angular, I send the token in the Authorization header as "Bearer" token.

Here is my ResourceServerConfiguration.java:

import org.springframework.context.annotation.Configuration;
import org.springframework.http.HttpMethod;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.oauth2.config.annotation.web.configuration.EnableResourceServer;
import org.springframework.security.oauth2.config.annotation.web.configuration.ResourceServerConfigurerAdapter;
import org.springframework.security.oauth2.config.annotation.web.configurers.ResourceServerSecurityConfigurer;

@Configuration
@EnableResourceServer
public class ResourceServerConfiguration extends ResourceServerConfigurerAdapter {
    private static final String RESOURCE_ID = "resource-server-rest-api";
    private static final String SECURED_READ_SCOPE = "#oauth2.hasScope('openid')";
    private static final String SECURED_WRITE_SCOPE = "#oauth2.hasScope('openid')";
    private static final String SECURED_PATTERN = "/**";
    @Override
    public void configure(ResourceServerSecurityConfigurer resources) {
        resources.resourceId(RESOURCE_ID);
    }
    @Override
    public void configure(HttpSecurity http) throws Exception {
        http.requestMatchers()
                .antMatchers(SECURED_PATTERN).and().authorizeRequests()
                .antMatchers(HttpMethod.POST, SECURED_PATTERN).access(SECURED_WRITE_SCOPE)
                .anyRequest().access(SECURED_READ_SCOPE);
    }
}

When calling the REST service my Angular UI gets HTTP Response 401 with the following error message:

DEBUG o.s.s.o.p.a.OAuth2AuthenticationProcessingFilter - Authentication request failed: error="invalid_token", error_description="Invalid access token: eyJraWQiOiIy.......
  1. Any idea why I am getting invalid_token?
  2. Does spring makes a call to Cognito to verify the token?
  3. I am NOT storing the token in my REST servic layer. Is this required?
  4. I enabled DEBUG using logging.level.root=DEBUG. But I do not see descriptive message in the output. How do i trouble shoot this issue?

Thanks in advance for any help in resolving this issue.