I tracked down the issue: your volume was on a host that went down hard a few weeks ago. We migrated all volumes from that host, but it appears yours fell through the cracks.
If a volume doesn’t exist on any host, our scheduler can’t place your app.
I’m consulting with the team on what our options are here, we should have a backup of your volume.
To get back up more quickly, if your data can be re-created easily, I’d recommend creating a new volume for your app. Placement should then be possible and your app will come back up. Sorry if this sounds insane, we value the data hosted in volumes by our users, just giving you a quick workaround if it turns out your data is disposable (it is for some of our users).