On level 3-4, there is a mistake in word ‘ziemia’, which is written incorrectly ‘ziemi’. ‘Ziemi’ could be Gen, Acc, or Loc, but the nominative form is ‘ziemia’. If you don’t believe me, see Polish Wikisłownik: https://pl.wiktionary.org/wiki/ziemia
À propos, your error reporting system is horrible. No one who has any idea of user-friendly system design would make a user go to a separate site, create a new topic on forum and watch a lot of suggestions that are absolutely useless (“Your topic is similar to…oh dear, we also have errors in Korean and Japanese, these are clearly relevant for someone reporting an error in Polish!”), instead of clicking on an icon right in the test, determining the context more accurately than most people can do from memory an hour or a day later.
And it is really weird that you deem it necessary to demand a separate login and mail activation for the forums while a user already has signed in to your main site. They’re both your own sites, after all.