Mavlink crashes when trying to connect over USB to Pixhawk

I have been using Mavlink to connect via USB to the nsh shell on my Pixhawk for a while now, but today I got a strange error. It tries to connect, but then I get an endless series of error messages:

Device /dev/tty.usbmodem1 reopened OK
Device /dev/tty.usbmodem1 is dead
Device /dev/tty.usbmodem1 reopened OK
Device /dev/tty.usbmodem1 is dead
Device /dev/tty.usbmodem1 reopened OK
Device /dev/tty.usbmodem1 is dead
Device /dev/tty.usbmodem1 reopened OK

Does anyone know what might be causing this and how to fix it? (The USB port and cable do work and I can connect to the Pixhawk over USB with QGroundControl with no issues.) I’m using macOS Sierra.

You’ve had it work before? Is QGC still running when you try to connect? Do you have any other USB/serial devices connected?

Yes, it has worked before, and no on QGC or other USB devices.

Sorry, I’m not sure. It seems like it might be a Sierra issue though.

The same issue on MacOS High Sierra 10.13.4.
Have you solved it already?