API Call from App Engine flexible to Firebase FCM failed - google-app-engine

Question

API Calls,e.g. to Firebase https://fcm.googleapis.com/fcm/send worked before switching from <env>vm</env> appengine-web.xml to
env: flex app.yaml flexible.
When deploying with mvn appengine:deploy everything is fine, but when an API call gets fired, which worked before, i get the exception below:
Exception in thread "Thread-14" com.google.apphosting.api.ApiProxy$CallNotFoundException:
Can't make API call urlfetch.Fetch in a thread that is neither the original request thread nor a thread created by ThreadManager
at com.google.apphosting.api.ApiProxy$CallNotFoundException.foreignThread(ApiProxy.java:800)
at com.google.apphosting.api.ApiProxy.makeSyncCall(ApiProxy.java:112)
at com.google.appengine.api.urlfetch.URLFetchServiceImpl.fetch(URLFetchServiceImpl.java:40)
at com.google.api.client.extensions.appengine.http.UrlFetchRequest.execute(UrlFetchRequest.java:74)
at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:981)
at com.qweez.flexenv.service.GameCounterService.run(GameCounterService.java:80)
it crashes when calling request.execute() below:
import com.google.api.client.http.ByteArrayContent;
import com.google.api.client.http.GenericUrl;
import com.google.api.client.http.HttpRequest;
import com.google.api.client.http.HttpResponse;
import java.io.IOException;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
import java.util.logging.Logger;
// stuff
try {
HttpRequest request = HttpRequestUtil.getHttpRequestFactory().buildPostRequest(new GenericUrl(GlobalConfig.MESSAGING_SERVER_URL), ByteArrayContent.fromString("application/json", requestBody));
HttpResponse response = request.execute();
//logger.warning(LOG_TAG+" ++++++ response status=" + response.getContent());
} catch (IOException e) {
logger.warning(LOG_TAG+" sent messge to topic error: " + e.getMessage());
}
the helper class httpRequestUtil looks like this
import com.google.api.client.extensions.appengine.http.UrlFetchTransport;
import com.google.api.client.http.HttpHeaders;
import com.google.api.client.http.HttpRequestFactory;
import com.google.api.client.http.HttpRequestInitializer;
import com.google.api.client.http.HttpTransport;
import java.io.IOException;
public class HttpRequestUtil {
private static HttpTransport httpTransport;
private static HttpRequestFactory requestFactory;
static {
httpTransport = UrlFetchTransport.getDefaultInstance();
requestFactory = httpTransport.createRequestFactory(new HttpRequestInitializer() {
#Override
public void initialize(com.google.api.client.http.HttpRequest httpRequest) throws IOException {
HttpHeaders hh = new HttpHeaders();
hh.setAuthorization(GlobalConfig.SERVER_KEY);
hh.setContentType("application/json");
httpRequest.setHeaders(hh);
}
});
}
public static HttpRequestFactory getHttpRequestFactory(){
return requestFactory;
}
Is there something deprecated now?
Or what's the issue here now?
Any help very appreciated. Thanks!