When I want to duplicate a session the label code is not editable, and there isn't a new label created. Therefore I cannot change the description of the session, because then also the original is changed :( .
How can I fix this?

(btw: The same works fine when duplicating reports!)

When you duplicate a session, then the label code is empty and you have to add/change the existing description and save, the tools will automatically generated the session desc's label code. The label code field is not editable field. If you change the label's description from ttadv1140m000 (Labels session) or from the single-occ of sessions' ttadv2100s000 then the Session Description is also changed.