After correct setup of ContentTranslation extension, when entering Special:ContentTranslation I got following error:

[a55ecb09724b28a6c22b34be] /mediawiki/index.php?title=Specjalna:ContentTranslation&campaign=specialcx Error from line 93 of E:\mediawiki-core\extensions\ContentTranslation\specials\SpecialContentTranslation.php: Class 'ContentTranslation\Translation' not found

Backtrace:

#0 E:\mediawiki-core\extensions\ContentTranslation\specials\SpecialContentTranslation.php(155): SpecialContentTranslation->isExistingTranslation()
#1 E:\mediawiki-core\includes\specialpage\SpecialPage.php(522): SpecialContentTranslation->execute(NULL)
#2 E:\mediawiki-core\includes\specialpage\SpecialPageFactory.php(578): SpecialPage->run(NULL)
#3 E:\mediawiki-core\includes\MediaWiki.php(287): SpecialPageFactory::executePath(Title, RequestContext)
#4 E:\mediawiki-core\includes\MediaWiki.php(851): MediaWiki->performRequest()
#5 E:\mediawiki-core\includes\MediaWiki.php(523): MediaWiki->main()
#6 E:\mediawiki-core\index.php(43): MediaWiki->run()
#7 {main}

This occurs because of some issue with autoloading. When I added manually require_once __DIR__.'/../includes/Translation.php';, everything works. composer dumpautoload or install nor update doesn't help.