Thread:Project:Support desk/Browsing categories by letter

This template and similar ones on Wikipedia provide a convenient way to browse a category by letting it start on a particular numeral or letter. An example on this site is http://www.mediawiki.org/wiki/Category:All_extensions.

The template worked for me in MW 1.16, but it stopped doing so in MW 1.17 (for no apparent reason). Manually adding  (for M) to the URL achieves nothing unfortunately.

How do we go about enabling it again?