Trouble with Grails 3 spring security sample

Hello,
I’m desperately trying to get the Grails 3 spring security sample to work, but apparently some things changed in your API and the sample is out-of-date. When I follow all the instructions in the README, I get this exception: IllegalStateException thrown while decoding JWT token PublicKey cannot be null when using algorithm: SHA256withRSA · Issue #2 · auth0-samples/auth0-grails3-spring-security-mvc-sample · GitHub
I read here and there that we can’t use HS256 as a signature algorithm, so I changed it to RS256 in the client advanced settings and in auth0.properties, and I downloaded the PEM certificate to src/main/resources/WEB-INF in the project, but now I’m getting a completely different and weird exception on application startup:

ERROR org.springframework.boot.SpringApplication - Application startup failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'springSecurityFilterChain' defined in class path resource [org/springframework/security/config/annotation/web/configuration/WebSecurityConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.servlet.Filter]: Factory method 'springSecurityFilterChain' threw exception; nested exception is org.springframework.security.config.annotation.AlreadyBuiltException: This object has already been built
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:599)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1123)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1018)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:510)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:296)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:839)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:538)
	at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:118)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:766)
	at org.springframework.boot.SpringApplication.createAndRefreshContext(SpringApplication.java:361)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:307)
	at grails.boot.GrailsApp.run(GrailsApp.groovy:55)
	at grails.boot.GrailsApp.run(GrailsApp.groovy:374)
	at grails.boot.GrailsApp.run(GrailsApp.groovy:363)
	at grails.boot.GrailsApp$run.call(Unknown Source)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:133)
	at auth0.grails3.mvc.sample.Application.main(Application.groovy:14)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.servlet.Filter]: Factory method 'springSecurityFilterChain' threw exception; nested exception is org.springframework.security.config.annotation.AlreadyBuiltException: This object has already been built
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189)
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588)
	... 25 common frames omitted
Caused by: org.springframework.security.config.annotation.AlreadyBuiltException: This object has already been built
	at org.springframework.security.config.annotation.AbstractSecurityBuilder.build(AbstractSecurityBuilder.java:44)
	at org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration.springSecurityFilterChain(WebSecurityConfiguration.java:105)
	at org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration$$EnhancerBySpringCGLIB$$6570aebc.CGLIB$springSecurityFilterChain$3(<generated>)
	at org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration$$EnhancerBySpringCGLIB$$6570aebc$$FastClassBySpringCGLIB$$c7e1568.invoke(<generated>)
	at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228)
	at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:356)
	at org.springframework.security.config.annotation.web.configuration.WebSecurityConfiguration$$EnhancerBySpringCGLIB$$6570aebc.springSecurityFilterChain(<generated>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at org.springsource.loaded.ri.ReflectiveInterceptor.jlrMethodInvoke(ReflectiveInterceptor.java:1426)
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162)
	... 26 common frames omitted

Any idea how to fix this?