I re-opened again and found the blend mode had changed itself back to alpha and now it works fine. I saved a new branch and was able to recreate the error:
An error has occurred. See error log for more details.
Invalid path '/blend_mode'. No such element 'blend_mode' in 'LabelDesc'
Side note: If anyone could tell me how to get the font to look like this in the game I'd be very grateful. I have spent a great number of hours trying to make it work: