encountered JAVA com.google.apphosting.api.ApiProxy$CallNotFoundException when using geocoding in GCP app-engine

I need to call GeocodingApi.reverseGeocode from with a java app running in an GCP app-engine. My code looks like:

context = new GeoApiContext.Builder(new GaeRequestHandler.Builder())
.apiKey("").build();

LatLng location = new LatLng(posLat, posLong);
GeocodingApiRequest request = GeocodingApi.reverseGeocode(context, location);
GeocodingResult[] results = request.await();

Running this code gives the exception:

A 2020-05-14T17:42:52.557941036Z com.google.maps.errors.UnknownErrorException: Unexpected exception from 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

A 2020-05-14T17:42:52.559374731Z at com.google.maps.internal.GaePendingResult.await(GaePendingResult.java:128)

A 2020-05-14T17:42:52.559425995Z at com.google.maps.PendingResultBase.await(PendingResultBase.java:58)

A 2020-05-14T17:42:52.559438799Z at de.bit.daimler.vehicletripanalyzer.geocoding.VTA_GeoCode.getCityName(VTA_GeoCode.java:42)

A 2020-05-14T17:42:52.559526039Z at de.bit.daimler.vehicletripanalyzer.analyzer.TripAnalyzer.buildResponse(TripAnalyzer.java:50)

A 2020-05-14T17:42:52.559550842Z at de.bit.daimler.vehicletripanalyzer.analyzer.TripAnalyzer.analyseTrip(TripAnalyzer.java:43)

A 2020-05-14T17:42:52.559557289Z at de.bit.daimler.vehicletripanalyzer.rest.VTA_RestController.analyzeTrip(VTA_RestController.java:32)

A 2020-05-14T17:42:52.561769118Z at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

A 2020-05-14T17:42:52.561810065Z at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

A 2020-05-14T17:42:52.561891356Z at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

A 2020-05-14T17:42:52.561902134Z at java.lang.reflect.Method.invoke(Method.java:498)

I updated all my recources (especially google-dependencies) to the latest available versions. I also use the latest image for the GCP app engine. But still the error persists.

I guess that this issue is related to https://github.com/googlemaps/google-maps-services-java/issues/672, but there another language and API-call is used. Therefor I'm raising a new issue.

Help is urgently appreciated... Thanks a lot!