A Study Evaluating the Safety and Efficacy of GLPG5101 (19CP02) in Participants With Non-Hodgkin Lymphoma

This study is evaluating whether an experimental treatment called GLPG5101 helps to treat non-Hodgkin lymphoma (NHL) and if it is safe to use. This study will be carried out in 2 phases: * The first phase is to see which doses of GLPG5101 work best with the least number of side effects. * In the second phase, participants will receive the selected dose(s) based on the results in the first phase.

Key Inclusion Criteria: * Histologically confirmed diagnosis of one of the following NHL subtypes: DLBCL, FL grade 1, 2 or 3A, MZL, MCL, BL, PCNSL, DLBCL-RT, High Grade B-cell Lymphoma (HGBL), CLL/SLL * Relapsed or refractory disease * Presence of at least one measurable lesion according to the Lugano classification (except for PCNSL subjects ineligible for ASCT after induction therapy, Cohort 6b; and except for CLL/SLL subjects without a measurable lesion or a PET positive lesion will be eligible if they have splenomegaly (spleen size \>13 cm) and bone marrow infiltration with lymphoma) * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 (Participants with ECOG 2 must have serum albumin ≥ 3.4 gram/deciliter) * Adequate bone marrow function * Adequate renal, hepatic and pulmonary function * Women of childbearing potential must have a negative serum pregnancy test at screening and prior to the first dose of conditioning chemotherapy * Women of childbearing potential and all male subjects must agree to use highly effective methods of contraception and agree to remain on a highly effective method of contraception from the time of signing the informed consent form until at least 12 months after GLPG5101 infusion. Subjects must agree to not donate eggs or sperm during this period. Key Exclusion Criteria: * Selected prior treatments as defined in the protocol * History of another primary malignancy that requires intervention beyond surveillance or that has not been in remission for at least 3 years. (exceptions per protocol) * Toxicity from previous anticancer therapy that has not resolved to baseline levels or to ≤ Grade 2 * Active central nervous system (CNS) involvement (lesion on contrast-enhanced CT/MRI brain, malignant B cells in CSF) by disease under study (exceptions per protocol) * Clinically significant cardiac disease * Primary immunodeficiency * Stroke or seizure within 6 months of screening * History of autoimmune disease requiring systemic immunosuppression or disease modifying treatment within 28 days before screening * Infection with human immunodeficiency virus (HIV), active hepatitis B or active hepatitis C virus * Systemic fungal, bacterial, viral, or other infection that is not controlled