I just had to do this - again. After collecting the requisite scar tissue on this issue in the past, this is what works for me:
1) Highlight the serial text in the sticky, then do a right click and select “copy”. If you do this carefully, you can avoid capturing the infuriating additional space on the end. If you can’t seem to capture the serial alone (you keep getting the space) do the copy anyway…
2) In the Carrara nuisance ..er.. reminder panel, delete the entire old serial number, leaving your cursor in the serial field.
3) On Windows system, do a CTRL-V. (Or you can right click and select “paste”). (Mac users, maybe you can tell us the Mac equivalents.)
4) Click into the end of the field or select the END key. If the dastardly blank character is at the end of the serial number string, delete it.
5) I can’t remember the name on the button, I think it might have been “update”, click it - voila, your serial should have been accepted.
Hope this helps, it worked for me just now…