I was in the same situation after upgrading my phone’s operating system. This fairly unhelpful message appeared when I tried to open the Revolut app:
I could dismiss the message and proceed to the login screen, but then I was caught in a cycle of verifying my phone number followed by another error message:
As the first message suggested, uninstalling and reinstalling the app resolved the problem and I haven’t had any issues since.
In case any of the team see this, the technical details are as follows:
Handset: OnePlus 6
Android version: 9
OxygenOS version: 9.0
App version: 5.17