Hello itoldusoandso,
We're glad you thought to ask Apple Support Communities about the Mail app quitting when changing the order of your Inboxes. If you haven't already, restart your iPad to see if that helps. Restarting can resolve many odd issues: Restart your iPad
If that doesn't help, do you use a VPN or security software? If so, temporarily disable or remove those to see if the behavior continues. If so, you'll want to work with the VPN or security software app developer: How to contact an app developer
If you don't use those or the issue continues after they're removed/disabled, remove the Mail app and reinstall it to see if that helps. Please note, only complete this step if you do not have any POP accounts as it can cause loss of those emails: Delete built-in Apple apps from the Home screen on your iOS 14 or iPadOS device or Apple Watch
Keep us posted on the results.
Take care.