AVAudioEngine startAndReturnError is now failing

I have a keyboard in my iOS Morse Code app that has always been able to play audio via AVAudioEngine. Recently it has been failing to produce audio. I see that startAndReturnError: is now failing with this error:

Error Domain=com.apple.coreaudio.avfaudio Code=268435459 "(null)" UserInfo={failed call=err = PerformCommand(*outputNode, kAUInitialize, NULL, 0)}

What's going on? Have keyboards lost the ability to play audio? Here's how I set things up:

		_engine = [AVAudioEngine new];
		_prefs = [[NSUserDefaults alloc] initWithSuiteName:kSharedAppGroupID];
		
		AVAudioMixerNode*		mainMixerNode = _engine.mainMixerNode;
		AVAudioOutputNode*		outputNode = _engine.outputNode;
		AVAudioFormat*			format = [outputNode inputFormatForBus:0];
		AVAudioFormat*			inputFormat = [[AVAudioFormat alloc] initWithCommonFormat:AVAudioPCMFormatFloat32 sampleRate:44100 channels:1 interleaved:NO];
		
		self.srcNode = [[AVAudioSourceNode alloc] initWithRenderBlock:^OSStatus(BOOL* _Nonnull isSilence, const AudioTimeStamp* _Nonnull timestamp, AVAudioFrameCount frameCount, AudioBufferList* _Nonnull outputData) {
// This block builds the data, but is never called, so it is not the culprit.
}];

		[_engine attachNode:self.srcNode];
		[_engine connect:self.srcNode to:mainMixerNode format:inputFormat];
		[_engine connect:mainMixerNode to:_engine.outputNode format:nil];
		[_engine prepare];