Inclusion criteria  Signed and dated written informed consent. Male or female patients who are 18-50 years (inclusive) of age at time of consent.  Diagnosis of schizophrenia utilizing Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-5).  -- Patients must be clinically stable and in the residual (non-acute) phase of their illness with no hospitalization or increase level of care due to worsening of schizophrenia in the past 12 weeks or no uncontrolled positive symptoms.  Patients should have functional impairment in day-to-day activities per investigator judgement. Patients maintained on current antipsychotic treatment for at least 12 weeks and on current dose for at least 35 days prior to randomization. Patients with any other concomitant psychoactive medications (except for anticholinergics) need to be maintained on same drug for at least 12 weeks and on current dose/ regimen for at least 35 days prior to randomization. Women of childbearing potential must use highly effective methods of birth control. Have a study partner who interacts with the patient on a regular basis. Further inclusion criteria apply.  Exclusion criteria  Patient with current DSM-5 diagnosis other than Schizophrenia. Cognitive impairment due to other causes, or patients with dementia. Severe movement disorders. Any suicidal behavior in past year or suicidal ideation in the past 3 months. History of moderate or severe substance use disorder within the last 12 months prior to informed consent. Positive urine drug screen. Patients who were treated with Clozapine, stimulants, ketamine or electroconvulsive therapy within 6 months prior to randomization. Current participation in any investigational drug trial. Cognitive Remediation Therapy within 12 weeks prior to screening. Initiation or change in any type or frequency of psychotherapy within 12 weeks prior to randomization. Any clinically significant finding or condition that would jeopardize the patient´s safety while participating in the trial or their capability to participate in the trial. Haemoglobin (Hb) below lower limit of normal . History of haemolytic anaemia, red blood cell (RBC) membrane diseases, known Glucose-6-phosphate dehydrogenase deficiency, anaemia of any cause or patients planning to donate blood. Severe renal impairment. Indication of liver disease. Any documented active or suspected malignancy or history of malignancy within 5 years. Women who are pregnant, nursing, or who plan to become pregnant while in the trial.  Further exclusion criteria apply.