I have the same issue on my iPhone 7 running IOS 13.3.1 whilst attempting to "Upgrade to Enpass Lite" and later on in the same screen whilst attempting to "Use Enpass Premium" on other devices. Here is my chain of events;
I hadn't bought the software yet, thus I decided to buy it:
I choose the option "Upgrade to Enpass Lite" in the blue bar at the bottom of the screen
I get the "Congrats, you are eligible to update to..."
I choose "Register with email".
Enter my mail address and choose next ">"
Now the app just quits/crashes.
I restart the app and perform steps 1 and 2 again
Now I choose "Continue with Apple ID"
I verify my Apple Account and continue.
Now the app again just quits.crashes
OK, I'm annoyed now, but being developer myself I decided to delete and re-install the app to see what happens.
I reinstall the app and during the initial start-up I'm offered to register myself and buy the premium license.
I do this, and it is successul.
I'm now able to synchronise all my data from the iCloud (created on my Mac) and the app works.
But now I want to enable the license across all my devices, so I choose : "Use Enpass Premium on other devices." in the blue bar at the bottom of the screen.
A similar screen appears, but now it says ""Congrats, you are eligible to use full version of Enpass across all your devices"
I choose "Register with email".
Enter my mail address and choose next ">"
Now the app just quits/crashes.
I restart the app and perform steps 1 and 2 again
Now I choose "Continue with Apple ID"
I verify my Apple Account and continue.
Now the app again just quits.crashes
Clearly something is wrong in this part of the app, or perhaps it's my iPhone itself.
So, I choose to be even more thorough and Factory Reset my phone completely;
I factory reset the phone.
Re-enable the app.
The app works.
I choose : "Use Enpass Premium on other devices." in the blue bar at the bottom of the screen again
Run through the procedure, but the app still crashes.
Officially I'm now stuck and clearly need some assistance from support to help me on this subject.
Hopefully my detailed error description is helpful.
Hope to hear from you soon.