The primary goal of this study is to assess the safety of two types of oral anticoagulants in patients with ESKD on hemodialysis with non-valvular atrial fibrillation (NVAF). The novel FXa inhibitor apixaban (at a reduced dose of 2x 2.5 mg/day) will be compared to the vitamin-K antagonist (VKA) phenprocoumon (target range: International Normalized Ratio (INR) 2.0-3.0) regarding bleeding rates during chronic administration for prevention of stroke or systemic embolism.