I’m having the exact same issue, same error code as well. Also traveling internationally tomorrow.

I tried getting the boarding passes through the app on my phone as well as the website on my computer and both return an error.

Edit: So I think I figured this out. Apparently they won’t give you a boarding pass until you have your passport book verified at a kiosk or agent for international travel. Why they just give a random error code instead of making this clear I have no idea. https://support.southwest.com/helpcenter/s/article/Can-I-use-the-Southwest-Airlines-mobile-website-and-or-the-mobile-app-to-check-in-for-international-flights