Adding more information on this problem. The UI tests are in a separate project in the same workspace. We split this way to reduce the amount of conflicts in the project file since the xcuitests are written by a separate team and the app project gets so many updates per day. I noticed that if I add the UI test target in the app scheme without using test plans I can run the ui tests in parallel many times in a row, but if I convert to test plans or if I keep the UI tests in a separate scheme using test plans I can run the ui tests in parallel only once and then I get this error on the subsequent tries. The only way around it is to delete the simulator and recreate it.