This went under the radar for a few days, but MediaWiki 1.20wmf11
brought some major changes to plural handling, thanks to the new CLDR
plural parser. Careful release notes reader could have spotted it here:
https://www.mediawiki.org/wiki/MediaWiki_1.20/wmf11 (core changes).
Niklas has just added some (hopefully temporary) overrides to restore
the old behaviour and fix
https://bugzilla.wikimedia.org/40251 : you
should report on the bug if your language has the same problem.
<https://translatewiki.net/wiki/Plural/Comparison_of_plural_rules_in_various_databases>
lists the differences between CLDR and MediaWiki rules and
<https://translatewiki.net/wiki/Thread:Support/PLURAL_keyword_for_languages_without_grammatical_plural_forms>
has some more discussion.
Nemo
P.s.: 1.20wmf11 has been enabled everywhere on the 12th and the switched
on and off a few times on en.wiki but it seems it will stay now despite
some other bugs, check [[Special:Version]].