Here is the output from my app during the error. It happens on the very first frame captured by Twilio Video, after ARKit has been running for a few seconds already. It only happens when using the H264 codec. I've opened a support ticket with them as well.
```
2017-10-02 23:41:17.537878-0700 App DEV[2776:550759] [] nw_connection_get_connected_socket 33 Connection has no connected handler
2017-10-02 23:41:17.537904-0700 App DEV[2776:550759] TCP Conn 0x1c0363900 Failed : error 0:61 [61]
2017-10-02 23:41:17.775368-0700 App DEV[2776:550539] XMachServer.cpp:106:RequestPortDeathNotification: AudioSessionClient is deallocating someone's notification port! (0x19b07)
2017-10-02 23:41:17.806318-0700 App DEV[2776:550565] [] <<<< AVCaptureDevice >>>> _registerServerConnectionDiedNotification_block_invoke: (pthread:0x16f99f000) ServerConnectionDied
2017-10-02 23:41:17.807316-0700 App DEV[2776:550549] [] <<<< CompressionSessionRemote >>>> vtcsr_dequeueAllPendingFramesAndCallbackClientForEach_block_invoke: dequeue failed (-12912)
2017-10-02 23:41:17.807502-0700 App DEV[2776:550769] [] VTCompressionSessionRemote_GetPixelBufferPool signalled err=-12912 (kVTVideoEncoderMalfunctionErr) (Lost server connection) at /BuildRoot/Library/Caches/com.apple.xbs/Sources/EmbeddedCoreMedia/EmbeddedCoreMedia-2180.7/Celeste/Sources/VideoToolboxRemote/VTCompressionSession_Remote.c:970
2017-10-02 23:41:17.807626-0700 App DEV[2776:550769] [] <<<< CompressionSessionRemote >>>> VTCompressionSessionRemote_GetPixelBufferPool: creating pixel buffer pool failed -12912
2017-10-02 23:41:18.566697-0700 App DEV[2776:550549] [] nw_connection_get_connected_socket 34 Connection has no connected handler
2017-10-02 23:41:18.566725-0700 App DEV[2776:550549] TCP Conn 0x1c417b600 Failed : error 0:61 [61]
2017-10-02 23:41:18.896441-0700 App DEV[2776:550565] [] <<<< AVCaptureSession >>>> -[AVCaptureSession _handleServerConnectionDiedNotification]: (0x1c401ca40)(pthread:0x16f99f000) ServerConnectionDied
2017-10-02 23:41:18.922468-0700 App DEV[2776:550429] [Session] Session did fail with error: Error Domain=com.apple.arkit.error Code=102 "Required sensor failed." UserInfo={NSLocalizedFailureReason=A sensor failed to deliver the required input., NSUnderlyingError=0x1c524d470 {Error Domain=AVFoundationErrorDomain Code=-11819 "Cannot Complete Action" UserInfo={NSLocalizedDescription=Cannot Complete Action, NSLocalizedRecoverySuggestion=Try again later.}}, NSLocalizedRecoverySuggestion=Make sure that the application has the required privacy settings., NSLocalizedDescription=Required sensor failed.}
2017-10-02 23:41:18.993403-0700 App DEV[2776:550565] [] <<<< AVCaptureDevice >>>> _registerServerConnectionDiedNotification_block_invoke: (pthread:0x16f99f000) ServerConnectionDied
2017-10-02 23:41:18.993972-0700 App DEV[2776:550539] [] <<<< CompressionSessionRemote >>>> vtcsr_dequeueAllPendingFramesAndCallbackClientForEach_block_invoke: dequeue failed (-12912)
2017-10-02 23:41:22.031352-0700 App DEV[2776:550769] [] nw_connection_get_connected_socket 35 Connection has no connected handler
2017-10-02 23:41:22.031409-0700 App DEV[2776:550769] TCP Conn 0x1c0362e80 Failed : error 0:61 [61]
2017-10-02 23:41:23.669098-0700 App DEV[2776:550565] [] <<<< AVCaptureSession >>>> -[AVCaptureSession _handleServerConnectionDiedNotification]: (0x1c401ca40)(pthread:0x16f99f000) ServerConnectionDied
2017-10-02 23:41:23.672653-0700 App DEV[2776:550565] [] nw_connection_get_connected_socket 36 Connection has no connected handler
2017-10-02 23:41:23.672679-0700 App DEV[2776:550565] TCP Conn 0x1c417ec00 Failed : error 0:61 [61]
2017-10-02 23:41:23.783749-0700 App DEV[2776:550429] 30: AudioSessionSetActive_Priv is passing a serverPID of 0 to CheckRPCError!
```
It was actually `sessionDidFailWithError` that was called.
I'm going to try creating a simpler app and see if I can reproduce with nothing else going on.