I haven't observed this in the past few days on translatewiki.net. Is it still observed on Wikimedia wikis? URL now gives a correct feed.

Works for me too. Please reopen if this is still / again a problem.

[13-Feb-2014 21:56:48] Fatal error: __clone method called on non-object at /usr/local/apache/common-local/php-1.23wmf14/extensions/LiquidThreads/api/ApiFeedLQTThreads.php on line 81
Server: mw1125
Method: GET
URL: http://www.mediawiki.org/w/api.php?action=feedthreads&type=replies%7Cnewthreads&talkpage=Talk%3AHow+to+contribute&feedformat=atom
Cookie: $Version=1; mediawikiwikiSession=fcd759255d24171b6027b18a128484fc; $Path=/; mediawikiwikiUserID=deleted; $Path=/; mediawikiwikiToken=deleted; $Path=/; forceHTTPS=deleted; $Path=/; mediawikiwikiLoggedOut=1388268620; $Path=/; stopMobileRedirect=true; $Path=/
Backtrace:
#0 /usr/local/apache/common-local/php-1.23wmf14/extensions/LiquidThreads/api/ApiFeedLQTThreads.php(81): ApiFeedLQTThreads::createFeedItem()
#1 /usr/local/apache/common-local/php-1.23wmf14/extensions/LiquidThreads/api/ApiFeedLQTThreads.php(67): ApiFeedLQTThreads->createFeedItem(Object(stdClass))
#2 /usr/local/apache/common-local/php-1.23wmf14/includes/api/ApiMain.php(861): ApiFeedLQTThreads->execute()
#3 /usr/local/apache/common-local/php-1.23wmf14/includes/api/ApiMain.php(362): ApiMain->executeAction()
#4 /usr/local/apache/common-local/php-1.23wmf14/includes/api/ApiMain.php(333): ApiMain->executeActionWithErrorHandling()
#5 /usr/local/apache/common-local/php-1.23wmf14/api.php(76): ApiMain->execute()
#6 /usr/local/apache/common-local/w/api.php(3): require('/usr/local/apac...')
#7 {main}

Has something changed in our cookie handling in 1.23wmf14? Those look a bit strange