I'm currently working on creating my own transactional templates. Sometimes it seems like the templates save fine, but other times I just get a red box with no text in it that appears in the top bar of the page. I'm trying to figure out if this is an issue on my end or yours.
**It does work sometimes, but it is very spotty.
It seems like when it doesn't worker there is an error in the console of the browser that says worker-html.js could not load or something similar to that.
Try changing the "Template Name".
I had the same problem and the displayed error message only said that it was not possible to save and I should try again. I checked my network tab, saw a 400 status in the response saying that the name I had chosen was already in use. After changing the name, the problem was solved.
Why Sendgrid doesn't show that message I don't know.
EDIT on April 27, 2018: Sendgrid now shows a message saying that the name already exists