About [[MediaWiki:Listfiles-latestversion-no/lad]]
I randomly get "Saving the translation failed: Creating new empty pages is not allowed." with some of the translation. I got it with "Paste source text" as well.
Make any edit, save the message, reedit the message undoing the edit you made and save then. Now the message should be saved with the pasted source text.
Working in the new translation editor, I also cannot get an empty message to save, because the "Save the translation" submit button will not work - it appears in a pale rather than a bold blue colour. I tested this by saving something and then trying to blank the message, as the advice above suggests. What does work for me is to type the string and saving that. renders as a blank space.
In the old translation editor, however, I can save an empty message.