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.