I’m afraid that the Monzo app isn’t fully compatible with the iOS 14 beta just yet.
Our engineers are aware of a crash when the Monzo app is put into the background that may affect your ability to login.
We’re expecting that we’ll have this fixed within the next 2 weeks.
In the meantime, to restore access to your app, could you try reverting back to the latest public release of iOS 13 and log back into the Monzo app?
Alternatively, if you’ve got another device like a tablet, you should be able to log into that device as long as it’s not running iOS 14.
I’ve asked the team to look into the workaround mentioned by @Chapuys 
Thank you 