Before following your suggestions, I wanted to attach the output of "braser --debug", but the funny thing is that I can't reproduce the error any more.
But I made an interesting discovery. It seems that you can't copy an element of the brasero content list but you can paste to the content list. (the list which shows you which (audio) files were already added to the project) Actually this behaviour is somewhat logical, who wants to have the same track several times on his disc? (I did)
So this time brasero tried to import "gpg -o - --export 428D7C01 | sudo apt-key add -" on Ctrl-V because that was the last string I had copied. (from the manuel you linked)
So it seems that it was whatever I had copied to my clipboard the other day what crashed brasero.
My proposal is to check the clipboard content before pasting and adding it to the project. (only files allowed)
Please let me know if I can help you in other ways with this bug...