XPages elements, for example, xpViewCategories.xsp, are case sensitive when used in Web URLs.
For example, these two URLs are treated differently by the Domino server. The second URL will produce an error because it is not considered a valid URL due the the use of all lower-case letters.
The support technote Are links that contain an 'XPage' case-sensitive?
provides additional information, including a verification that the case-sensitivity is working as designed due to the underlying Java technology that is the basis for XPages.