Updating description of concepts in a module fails

In my module I want to update the descrition of a concept like that:

            ConceptDescription cdesc = new ConceptDescription();
            cdesc.setDescription(conceptDescription);
            cdesc.setLocale(loc_en);
            
            Collection<ConceptDescription> cdesc_col = new HashSet<ConceptDescription>();
            cdesc_col.add(cdesc);
            
            Concept con = conceptService.getConceptByName(conceptNameEn);
            con.setDescriptions(cdesc_col);

            conceptService.saveConcept(con);

Saving new concepts works fine, but not updating existing ones. I am not getting an error message. The concept is just not updated. What am I doing wrong ?