validation error detected

[wlo]

I use keycloak-4.8.3.Final and try to add SMS-Auth via AWS-SNS. I get the following in the keycloak log:
http-outgoing-6 >> "POST / HTTP/1.1[\r][\n]" http-outgoing-6 >> "Host: sns.eu-west-1.amazonaws.com[\r][\n]" http-outgoing-6 >> "Authorization: AWS4-HMAC-SHA256 Credential=YYY/20190219/eu-west-1/sns/aws4_request, Signed Headers=amz-sdk-invocation-id;amz-sdk-retry;host;user-agent;x-amz-date, Signature=XXX[\r][\n]" http-outgoing-6 >> "X-Amz-Date: 20190219T091006Z[\r][\n]" http-outgoing-6 >> "User-Agent: aws-sdk-java/1.11.173 Linux/4.12.14-25.25-default OpenJDK_64-Bit_Server_VM/25.181-b13/1.8.0_181 [\r][\n]" http-outgoing-6 >> "amz-sdk-invocation-id: xxx-yyy-xxx-xxx-yyy[\r][\n]" http-outgoing-6 >> "amz-sdk-retry: 0/0/500[\r][\n]" http-outgoing-6 >> "Content-Type: application/x-www-form-urlencoded; charset=utf-8[\r][\n]" http-outgoing-6 >> "Content-Length: 216[\r][\n]" http-outgoing-6 >> "Connection: Keep-Alive[\r][\n]" http-outgoing-6 >> "[\r][\n]" http-outgoing-6 >> "Action=Publish&Version=2010-03-31&PhoneNumber=00441234567890&MessageAttributes.entry.1.Name=AWS.SNS.SMS.Se nderID&MessageAttributes.entry.1.Value.DataType=String&MessageAttributes.entry.1.Value.StringValue=HomeOffice" http-outgoing-6 << "HTTP/1.1 400 Bad Request[\r][\n]" http-outgoing-6 << "x-amzn-RequestId: 21e5ac27-4464-5d70-a49f-f8c28e4a407b[\r][\n]" http-outgoing-6 << "Content-Type: text/xml[\r][\n]" http-outgoing-6 << "Content-Length: 352[\r][\n]" http-outgoing-6 << "Date: Tue, 19 Feb 2019 09:10:06 GMT[\r][\n]" http-outgoing-6 << "[\r][\n]" http-outgoing-6 << HTTP/1.1 400 Bad Request http-outgoing-6 << x-amzn-RequestId: 21e5ac27-4464-5d70-a49f-f8c28e4a407b http-outgoing-6 << Content-Type: text/xml http-outgoing-6 << Content-Length: 352 http-outgoing-6 << Date: Tue, 19 Feb 2019 09:10:06 GMT http-outgoing-6 << "<ErrorResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/">[\n]" http-outgoing-6 << " <Error>[\n]" http-outgoing-6 << " <Type>Sender</Type>[\n]" http-outgoing-6 << " <Code>ValidationError</Code>[\n]" http-outgoing-6 << " <Message>1 validation error detected: Value null at 'message' failed to satisfy constraint: Member must not be null</Message>[\n]" http-outgoing-6 << " </Error>[\n]" http-outgoing-6 << " <RequestId>21e5ac27-4464-5d70-a49f-f8c28e4a407b</RequestId>[\n]" http-outgoing-6 << "</ErrorResponse>[\n]"
and followed by this message:
2019-02-19 10:10:06,719 ERROR [six.six.keycloak.authenticator.KeycloakSmsAuthenticatorUtil] (default task-26) Fail to send SMS : com.amazonaws.services.sns.model.AmazonSNSException: 1 validation err or detected: Value null at 'message' failed to satisfy constraint: Member must not be null (Service: AmazonSNS; Status Code: 400; Error Code: ValidationError; Request ID: 21e5ac27-4464-5d70-a49f-f8c 28e4a407b)
I have added the client-id and the client-secret in the authentication flow. Do you know what is missing here?