From my talk page on mediawiki.org:
Special:ProtectedTitles doesn't work in some wikis in MediaWiki 1.18wmf1. It only gives a bunch of error messages.
Internal error
Non-string key given
Backtrace:
#0 /usr/local/apache/common-local/php-1.18/includes/GlobalFunctions.php(1173): MessageCache->get(NULL, true, Object(LanguageFi))
#1 /usr/local/apache/common-local/php-1.18/includes/GlobalFunctions.php(1297): wfMsgGetKey(NULL, true, Object(LanguageFi), false)
#2 /usr/local/apache/common-local/php-1.18/languages/Language.php(644): wfMsgExt(NULL, Array)
#3 /usr/local/apache/common-local/php-1.18/languages/Language.php(664): Language->getMessageFromDB(NULL)
#4 /usr/local/apache/common-local/php-1.18/languages/Language.php(997): Language->getMonthName(false)
Observed on fiwiki, lawiki, trwiki.
Version: unspecified
Severity: normal