There are no known issues with using upper characters in passwords. Could you please provide us with a more detailed explanation of the exact behavior of Sitefinity. When you enter the user credentials, do you get redirected to the same Login page or is there an error message? Have in mind that you need to assign this user to a role that has permissions for CMS Access. One such role is Administrators.
We are looking forward to your answer.
All the best,
the Telerik team