Xcode 10.2.1 Server can't start test runner for iOS 11.4 Simulator

On May 31 I switched my UI test environment as follows:


BEFORE:

- iPhone SE (Simulator) with iOS 11.4

- iPad (6th Gen, Simulator) with iOS 12.2

- iPhone 8 (real hardware) with iOS 12.4


AFTER:

- iPhone SE (Simulator) with iOS 11.4

- iPad (6th Gen, Simulator) with iOS 12.2

- iPhone Xs (Simulator) with iOS 12.2

- iPhone 8 (real hardware) with iOS 12.4


This seemed fine for one or two runs, but then the tests completely failed for iPhone SE (Simulator, iOS 11.4).
The problem is the same every single time:

  • Device Testing reports, "Unknown Status"
  • Issue reports, "Failed to establish communication with the test runner. The operation couldn't be completed. DTXProxyChannel error 1.
  • Log reports, Closing socket to testmanagerd for Sim iPhone SE. Failed to make test runner session: Channel disconnected.


Removed the iPhone Xs (Simulator, iOS 12.2) from the mix and testing worked again (i.e. no change to codebase being tested).

Server environment:

  • Xcode 10.2.1 (10E1001)
  • macMini (late 2014), 3Ghz i7, 16GB
  • macOS Mojave 10.14.5