That is Semantic MediaWiki throwing an error when setting the Has URL property  and failing validation to the URL data type. Protocol relative URL's are fine for links, but it's not a valid URL. I'll add some code to catch these and make them fully qualified. I'll have to guess so I'll just put http on them.
Thanks for letting me know Mark!
While poking around WikiApiary, I came across links to WikiHiero that
were highlighted with a warning, that when I moused over it that said
the "URIs of the form //www.mediawiki.org/wiki/Extension:WikiHiero are
One question: Why not?
Also, I'm curious who the warning is intended for. The phrase "not
allowed" seems prescriptive and I'm wondering where the prescriptions
come from and who they're intended for.
Imagination does not breed insanity. Exactly what does breed insanity
is reason. Poets do not go mad; but chess-players do.
-- G.K. Chesterson