Hello,
Maybe my answer is no longer relevant, but I'll save it here anyway.
I discovered, that after
webRtcEndpoint.gatherCandidates();
application recevied IceCandidates, but onIceCandidateFound callback didn't invoke.
It happens because application lost this messages without throwing any exceptions in class ParamsFlattener. There is a method "unflattedComplexType" which uses ParametrizedTypeImpl.
ParametrizedTypeImpl should be exported since jdk9, because it exists in sun.reflect package. This might work in kurento tutorial without exporting, because this part of ParamsFlattener is never called (please double check it, I'm not sure).
Try to add
--add-exports=java.base/sun.reflect.generics.reflectiveObjects=ALL-UNNAMED
to your jvmArgs