I got the same Exception when trying to access a Service via RestEasy. So in case someone else stumbles upon it:
as Andreas already mentioned, it often refers to a wrong URL.
In my case i could call my Service via Postman or the resteasy target with
final String path = "http://localhost:8080/repository/user";
e.g.
Client client = ClientBuilder.newBuilder().build();
WebTarget target = client.target("http://localhost:8080/repository/user");
Response response = target.request().get();
while using a Resteasy-Proxy i had to use
final String path = "http://localhost:8080/repository";
e.g.
ResteasyClient client = new ResteasyClientBuilder().build();
ResteasyWebTarget target = client.target(UriBuilder.fromPath("http://localhost:8080/repository"));
IUserTestRepository proxy = target.proxy(IUserTestRepository.class);