Unavailable External Storage (server-side) prevents Nextcloud App from working (maintenance mode)

### Steps to reproduce
1. Add a external WebDAV storage that is unavailable in a normal user account
2. Login with a newly installed Nextcloud App

### Expected behaviour
- Everything should work normally - when opening the unavailable storage, a helpful message should appear

### Actual behaviour
- No folder sync happens - instead the app displays a "maintenance mode" message.

### Can you reproduce this problem on https://try.nextcloud.com?
- Not possible as I cannot add external storage there.

### Environment data
Android version: 8.0.0

Device model: Samsung SM-A520F

Stock or customized system: Stock

Nextcloud app version: latest from Google Play (3.11.1)

Nextcloud server version: 18.0.3

Reverse proxy: nginx-proxy (Docker)