this very well might be a duplicate but can't find it now...
/wiki/Special:EntityData/Q13529975.json HttpError from line 359 of /srv/mediawiki/php-1.25wmf17/extensions/Wikidata/extensions/Wikibase/repo/includes/LinkedData/EntityDataRequestHandler.php: Failed to load entity Q13529975.
#0 /srv/mediawiki/php-1.25wmf17/extensions/Wikidata/extensions/Wikibase/repo/includes/LinkedData/EntityDataRequestHandler.php(378): Wikibase\Repo\LinkedData\EntityDataRequestHandler->getEntityRevision()
#1 /srv/mediawiki/php-1.25wmf17/extensions/Wikidata/extensions/Wikibase/repo/includes/LinkedData/EntityDataRequestHandler.php(227): Wikibase\Repo\LinkedData\EntityDataRequestHandler->showData()
#2 /srv/mediawiki/php-1.25wmf17/extensions/Wikidata/extensions/Wikibase/repo/includes/specials/SpecialEntityData.php(153): Wikibase\Repo\LinkedData\EntityDataRequestHandler->handleRequest()
#3 /srv/mediawiki/php-1.25wmf17/includes/specialpage/SpecialPage.php(383): Wikibase\Repo\Specials\SpecialEntityData->execute()
#4 /srv/mediawiki/php-1.25wmf17/includes/specialpage/SpecialPageFactory.php(581): SpecialPage->run()
#5 /srv/mediawiki/php-1.25wmf17/includes/MediaWiki.php(270): SpecialPageFactory::executePath()
#6 /srv/mediawiki/php-1.25wmf17/includes/MediaWiki.php(560): MediaWiki->performRequest()
#7 /srv/mediawiki/php-1.25wmf17/includes/MediaWiki.php(424): MediaWiki->main()
#8 /srv/mediawiki/php-1.25wmf17/index.php(46): MediaWiki->run()
if the entity does not exist, then don't think this is such an exceptional condition or error in the software. I don't think this should go in the exception logs, together with other exception log entries.