There is a failure when loading paraminfo of all modules from test.wikidata.org
I diagnosed a very similar problem on en wp beta, and it was the fancycaptchareload module causing the breakage.
very likely to be the same problem on the test wikis also, as we see the same 503 error for test.wikidata.org
however that module isnt deployed to test.wikipedia.org
Adding 'MediaWiki-API' as I cant find a 'fancycaptcha' extension in Phabricator.