I also came here troubleshooting silent failure of the action. I’m surprised that DT’s processing of the callback URL will fail if the body is blank (&text=[[body]]). Isn’t an empty text file a valid one?
I changed it to x-devonthink://createText?title=[[title]]&location=''&text=[[draft]] which saves the entire draft as the text. If it’s just a one-line draft, the send to DT will still succeed. I like to have a title line at the top of a text document anyway.