I found something weird and I have no idea how to solve this problem.
When you go to any tag page single_cat_title returns wrong category - in this case just one category called "Aktualności" (ID 1). You can see the wrong title in metatags (<title></title>) and above the content.
Please check it there: e.g. http://www.bise.pl/tag/promocje-w-bankach/ - there should be the title: "Promocje w bankach", not "Aktualności" which is regular category: http://www.bise.pl/category/aktualnosci/ - one of many.
Thank you in advance.