On the sign-in screen tap Forgot your password? and enter the email address of your account. We send you an email with a link.
Open that link on the device the app is installed on. The link leads back into the app, and that is where you set the new password. Opened on another device, it does not find the app.
The email has not arrived. Check your spam folder; the sender is noreply@citily.de. Also check that you entered the same address you registered with. For security reasons the app does not say whether an account exists for an address.
The link no longer works. It is valid for a limited time and only once. Simply request a new one.
The password must be at least eight characters long.