Audacity fails to start

Audacity v3.1.2
MacOS 10.14.6 Mojave, running on an Apple laptop
I’m trying to use a Behringer USB audio interface as the audio input device on this Mac laptop. With the Behringer plugged in, I can open System Preferences → Sound and see “USB Audio CODEC” available. If I select that device and close System Prefs, then click on Audacity in the dock, it never starts up. The icon bounces in the dock for more than a minute, then simply stops, with no error message, no splash screen, no Mac box saying the application unexpectedly quit. If I click on the icon in the dock again, nothing happens. If I right-click the icon, I have the option to Force Quit, which suggests that Audacity has partly started up but is hung. I can force-quit, but if I try to launch again, the startup fails again in the same way.

I can get Audacity to start up normally if I go back to System Prefs → Sound and select Built-In Microphone as the audio input device. Once Audacity is running, I can use its drop-down menu to select the USB Audio CODEC as the input device, and if I want, go back to the Sound pane and select the USB CODEC there as well. But why won’t Audacity start if the USB CODEC is selected ahead of time?