I have had exactly same problems. I create course, click save, then just get a "loading..." that will just hang forever if i let it. If i then go to the master course list page i can see my newly created courses, i can click and see them on that list page BUT if i click the course for more detail i then just get the darkened map screen with the "loading..." text.
Courses that i have created say 6 months ago seem to load up fine, its just the ones since last week that i have noticed problematic.
I have tried Garmin Connect on 3-4 different browsers both on a mac at home and a PC at work, all having the same issue so its not operating system or browser specific.