On the LOGIN SCREEN:
Problem: It's not clear where to put what information. There are two places that say "Login", and then there is not a recognizable username and password icon or hint to the user as to what and where to fill in the field. This relates to principles such as clearly defining input fields and anticipating the user's mental model.
Solution: Type Login Below at the top, then put an email icon with "Email" in grey inside the box and a lock icon with "Password" in grey inside the box.
Problem: First, there's not indicator of what characters are accepted for making such defined fields. But, mostly, it relates to the principle of asking for too much information to register when the user will probably be lazy and navigate away. It's too much work and information to input at once.
Solution: I really liked your example of getting the email address at the site to sign up and then later inputting your details in an email that comes to your inbox. I think that would be a good solution.
Problem: The responsive design looks like it was bad. "Home" is at the bottom left instead of at the top or in the Hamburger Menu, and the bottom section of the data runs off the side of the screen. It makes input or analysis impossible. So, I'd say responsive design would be the principle. The dates used also could be more specific.
Solution: Check and make sure the app looks correct on every device and works the same on every device.
The problem is that the fields aren't usable as defined fields for collecting any data - there are no fields to enter the data. It would only be text data, and it's vague at that. The font for the end date isn't at all consistent with the font for the rest of the screen and takes a larger hierarchy over the more important information displayed. This relates to principles of consistency, usefulness, being clear on input.
Solution: I would make data fields for the questions so that data could be collected, used and analyzed and then make the font of the end date look the same as the start date.