Serializing Json causes crash

Serializing a Json to FString sometimes seams to crash my game. It can happen randomly on many occasions, I suspect it having something to do with framerate drops or when something is spawned:

FString UArin_GameSave::GetTextToSave(
	TSharedPtr<FJsonObject> pJsonObject)
{
	FString ReturnedValue = "";

	//Validate JsonObject
	if (!pJsonObject.IsValid())
	{
		UE_LOG(LogTemp, Log, TEXT("UArin_GameSave::GetTextToSave: LoadedDataslotData is invalid"));
		return "";
	}

	//Prepare Serializing
	TSharedRef<TJsonWriter<>> Writer = TJsonWriterFactory<>::Create(&ReturnedValue);

	//Serialize
	FJsonSerializer::Serialize(
		pJsonObject.ToSharedRef(),	//Data that gets serialized
		Writer);					//The writer which outputs it to the ReturnedValue

	return ReturnedValue;	// <-- The crash report always reports this line
}

Does anyone have an idea what could be the cause?

The error message:
Access violation - code c0000005 (first/second chance not available)

An other theory of mine would be that the code somewhere somehow executes a SetField at the exact same time as the FJsonObject is serializing?

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Thanks