Had this error today when creating a custom authentication SPI for Keycloak

16:17:13,588 ERROR [org.keycloak.services.error.KeycloakErrorHandler] (default task-1) Uncaught server error: 
org.keycloak.authentication.AuthenticationFlowException

        at org.keycloak.authentication.AuthenticationProcessor.authenticateOnly(AuthenticationProcessor.java:913)
        at org.keycloak.protocol.oidc.endpoints.TokenEndpoint.resourceOwnerPasswordCredentialsGrant(TokenEndpoint.java:554)
        at org.keycloak.protocol.oidc.endpoints.TokenEndpoint.processGrantRequest(TokenEndpoint.java:187)
        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:498)
        at org.jboss.resteasy.core.MethodInjectorImpl.invoke(MethodInjectorImpl.java:140)
        at org.jboss.resteasy.core.ResourceMethodInvoker.internalInvokeOnTarget(ResourceMethodInvoker.java:509)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTargetAfterFilter(ResourceMethodInvoker.java:399)
        at org.jboss.resteasy.core.ResourceMethodInvoker.lambda$invokeOnTarget$0(ResourceMethodInvoker.java:363)
        at org.jboss.resteasy.core.interception.PreMatchContainerRequestContext.filter(PreMatchContainerRequestContext.java:358)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTarget(ResourceMethodInvoker.java:365)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:337)
        at org.jboss.resteasy.core.ResourceLocatorInvoker.invokeOnTargetObject(ResourceLocatorInvoker.java:137)
        at org.jboss.resteasy.core.ResourceLocatorInvoker.invoke(ResourceLocatorInvoker.java:106)
        at org.jboss.resteasy.core.ResourceLocatorInvoker.invokeOnTargetObject(ResourceLocatorInvoker.java:132)
        at org.jboss.resteasy.core.ResourceLocatorInvoker.invoke(ResourceLocatorInvoker.java:100)
        at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:443)
        at org.jboss.resteasy.core.SynchronousDispatcher.lambda$invoke$4(SynchronousDispatcher.java:233)
        at org.jboss.resteasy.core.SynchronousDispatcher.lambda$preprocess$0(SynchronousDispatcher.java:139)
        at org.jboss.resteasy.core.interception.PreMatchContainerRequestContext.filter(PreMatchContainerRequestContext.java:358)
        at org.jboss.resteasy.core.SynchronousDispatcher.preprocess(SynchronousDispatcher.java:142)
        at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:219)

This error happens because im not putting any userModel into Keycloak’s flowContext and adding a userModel solve this problem. Code can be seen here,

https://github.com/edwin/keycloak-password-encryptor/blob/master/src/main/java/com/edw/keycloak/spi/CustomKeycloakPasswordEncryptor.java#L93