EE-2463 When add agent to k8s with AGENT_SECRET, AGENT_SECRET should be configured on both side. otherwise, it will get unknown error. this error should be "agent already paired with another Portainer instance" (#6679)

api/http/handler/endpoints/endpoint_create.go

@@ -470,7 +470,7 @@ func (handler *Handler) createTLSSecuredEndpoint(payload *endpointCreatePayload,
 func (handler *Handler) snapshotAndPersistEndpoint(endpoint *portainer.Endpoint) *httperror.HandlerError {
 	err := handler.SnapshotService.SnapshotEndpoint(endpoint)
 	if err != nil {
-		if strings.Contains(err.Error(), "Invalid request signature") {
+		if strings.Contains(err.Error(), "Invalid request signature") || strings.Contains(err.Error(), "unknown") {
 			err = errors.New("agent already paired with another Portainer instance")
 		}
 		return &httperror.HandlerError{http.StatusInternalServerError, "Unable to initiate communications with environment", err}