I went into the depths of JSON-B and Yasson. And I landed at the class org.eclipse.yasson.internal.serializer.LocalDateTimeTypeSerializer which is in charge of serialization and deserialization of LocalDateTime objects. It inherits from org.eclipse.yasson.internal.serializer.AbstractDateTimeSerializer and returns a String. Using that org.glassfish.json.JSonGeneratorImpl#write(String) is called. And that method escapes the String with double quotes.
Seems it is not an easy task which can't be done just be changing the configuration, doesn't it?
I found a solution here: http://json-b.net/users-guide.html
I simply wrote a serializer which implements JsonbSerializer and JsonbDeserializer and added it to the JsonbConfig with "withSerializers" and "withDeserializers".
Maybe one thing to add: The interfaces must be declared on the class of the object you pass to the JsonbConfig. I tried to implement an AbstractSerializer<T> implementing these 2 interfaces and it got confused because it couldn't handle the generics.