This started today. Different browsers, as user or logged-out. Some part of the dependency system tries to load "wikibase.mobile" but fails.
From the JS error log (debug=true):
load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:13121 Error: Unknown dependency: wikibase.mobile Error: Unknown dependency: wikibase.mobile at sortDependencies (load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:11459) at resolveStubbornly (load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:11539) at Object.load (load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:12443) at Q21748978?debug=true:246 at startUp (load.php?debug=true&lang=en&modules=startup&only=scripts&skin=vector:9158) at HTMLScriptElement.script.onload.script.onreadystatechange (load.php?debug=true&lang=en&modules=startup&only=scripts&skin=vector:9184) logError @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:13121 handler @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:10965 fire @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:3148 fireWith @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:3260 fire @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:3267 track @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:10941 resolveStubbornly @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:11544 load @ load.php?debug=true&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=…:12443 (anonymous) @ Q21748978?debug=true:246 startUp @ load.php?debug=true&lang=en&modules=startup&only=scripts&skin=vector:9158 script.onload.script.onreadystatechange @ load.php?debug=true&lang=en&modules=startup&only=scripts&skin=vector:9184