Intravenous BI 836826 in combination with ibrutinib in relapsed/refractory Chronic Lymphocytic Leukemia (CLL) patients who have been pre-treated with at least one prior line of systemic therapy, and who are eligible for treatment with ibrutinib. Objectives of the trial are to determine the recommended Phase 2 dose of BI 836826, to investigate the number of patients with a complete response and to document the safety and tolerability of BI 836826 when given in combination with ibrutinib