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

Phase 1 Dose escalation phase: The dose escalation phase is designed to select the doses for dose expansion based on efficacy and safety outcomes. Three dose levels of GLPG5101 will be evaluated to determine the recommended phase 2 doses (RP2Ds). Participants with aggressive or indolent forms of Relapsed/Refractory B-cell Non-Hodgkin Lymphoma (r/r NHL) including follicular lymphoma (FL), marginal zone lymphoma (MZL), mantle cell lymphoma (MCL), and Diffuse large B-cell lymphoma (DLBCL) will be enrolled. In case any safety or efficacy differences based on histological subtype of the disease are observed, it may be decided to make decisions regarding dose escalation for a specific subtype(s) independently, upon recommendation by the Safety Review Committee (SRC). Hence the RP2Ds may be different for different subtypes. Phase 2 Dose expansion phase: After determination of the RP2Ds, the study continues with the dose expansion phase. Different doses deemed safe by the SRC within the same indication may be explored in the dose-expansion phase to help select the optimal dose for further development. During this phase, participants will be enrolled into separate disease cohorts as defined by their NHL subtype: * Cohort 1a: DLBCL second line or greater (2L+) * Cohort 1b: DLBCL 2L+ with secondary central nervous system lymphoma (SCNSL) * Cohort 2: High-risk first-line DLBCL * Cohort 3: Indolent B-cell NHL (FL and MZL third-line or greater \[3L+\]) * Cohort 4: MCL 2L+ * Cohort 5: BL 2L+ * Cohort 6a: PCNSL 2L+ * Cohort 6b: PCNSL first-line consolidation * Cohort 7: DLBCL-RT 2L+ * Cohort 8: CLL/SLL (r/r) Participants per disease cohort will be treated at the selected RP2Ds for that disease subtype.