1. Start an installer.
   I used a current Creator 3.5 package.
2. Proceed to the "Qt Account" page of the wizard.
3. If there is any, remove all data from the "Login" region.
4. In the "Sign-up" region, type an e-mail address and a short password.
5. Check "I accept..."
6. Try to click the "Next" button.
   You can't. It's disabled. There is no indication why this is the case.
7. Hover your mouse over the first password edit.
   You'll see a tooltip which lists the password rules.

If there is a rule that blocks the user from installing the product, the installer should at least clearly communicate this rule to the user. A tooltip hidden somewhere is not clear communication.
Eventually, the password rules will show up in a text below the input region, but usually they don't.