• Participant had a previous allogeneic HSCT as indicated for non-malignant (including inborn errors of metabolism, primary immunodeficiencies, haemoglobinopathies, and bone marrow failure syndromes) or hematological malignant disease or neuroblastoma.

• Participant has been clinically diagnosed with Grade II to IV aGvHD according to Harris et al. A biopsy of the involved organs with aGvHD is encouraged but not required.

• Participant has experienced failure of previous first-line aGvHD treatment (that is, SR-aGvHD), defined as:

‣ aGvHD progression within 3 to 5 days of therapy onset with \>=2 milligram per kilogram per day (mg/kg/day) of prednisone equivalent or

⁃ failure to improve within 5 to 7 days of treatment initiation with \>=2 mg/kg/day of prednisone equivalent or

⁃ incomplete response after greater than (\>) 28 days of immunosuppressive treatment including at least 5 days with \>=2 mg/kg/day of prednisone equivalent.

• Male or female participant who is \>=28 days and \<18 years of age and has a minimum body weight of 3.2 kilograms (kg) at the Screening Visit.

• Participant has an estimated life expectancy of \>28 days.

• Participant, if female and of childbearing potential, agrees to use a highly effective contraceptive measure starting at the Screening Visit and continuing throughout the entire trial period.

• Participant, if a fertile male, agrees to sexual abstinence or to use a condom during sexual activity with their female partner of childbearing potential or pregnant partner. Additionally, if their partner is a woman of childbearing potential (WOCBP), then their partner must use an additional highly effective contraceptive method during sexual activity starting at the Screening Visit and continuing throughout the entire trial period.

• A written informed consent of the participant's parent(s) / legal guardian(s) (and participant's assent, when applicable) has been obtained according to national regulations.