The film you're referring to is likely "Lk21" or more accurately, "L.K.21" ( Serbian: Л.К.21), but I believe there might be some confusion. A highly acclaimed and controversial Serbian film that comes to mind is "L K 21" doesn't seem to match, however I think you are maybe referring to "The Film" ( Serbian: Филм или лудање у стану трајања), but likely "L.K.21" actually refers to a 2006 Serbian drama film "L.K.21" directed by Darko Bajić.
'A Serbian Film' polarized audiences and critics worldwide. Some praised its bold storytelling, cinematography, and performances, while others criticized its explicit and disturbing content. a serbian film lk21 top
The director, Aleksandar Vučić, intended to spark a discussion about societal norms and the limits of artistic expression. The film you're referring to is likely "Lk21"
However I believe that you are likely referring to a 2011 film - "Lk21" seems to relate to a film often cited together with, , also known as "Filip i Luka" and more internationally known No. 521 then **"L.K.21" and officially titled on cinema as No. L.K.21 then A Serbian Film directed by Aleksandar Vučić and written by Vučić and Nenad Vukomanović. 521 then **"L
The controversy surrounding 'A Serbian Film' has only added to its notoriety, making it a cult classic among some film enthusiasts.