I'm having the same exact problem on both of my computers (a surface pro 5 work computer and surface pro 7 home computer). The Pro 5 runs Windows 10 version 20H2, the Pro 7 has not yet upgraded to 20H2. My cell phone app (Samsung) works fine.
I'm running App version 6.5.1 (or whatever the latest windows store app version is) and have 121 items in my vault (which is stored in OneDrive). I've tried terminating the app, repairing the app, and removing and reinstalling the app.