The aims of this study are to develop a safe ATRA and daratumumab combination suitable for clinical use and evaluation in subsequent randomized clinical trials. To this end, the maximum tolerated dose (MTD) of ATRA and daratumumab will be determined for patients with relapsed/refractory disease, who were treated with daratumumab but failed to achieve a partial response, or developed progressive disease during daratumumab treatment. This will be followed by a second part in which the investigators will examine the effectivity and toxicity profile of the combination at the MTD.