When using Special:PrefixIndex to search for subpages of a particular page, the recommended practice is to include a slash at the end of the query, e.g. [[Special:PrefixIndex/Main page/]]. However, this is currently b0rken in the API at least as of r40356: http://en.wikipedia.org/w/api.php?action=query&list=allpages&apprefix=Main-page/&aplimit=10 gives an error about "Main page/." being an invalid title.
This may have resulted from r40088; if so, changing the dummy character from '.' to 'x' should fix it. (OTOH, I could just be wrong about it).