I have found a fix for this issue, IF you are using one of the sign in options such as google, safari, etc. You need to log out of GraphAcademy, and sign in without using one of the options. type in your email and password, and log in. There seems to be an issue with those sign in options not properly logging people in. I am using google chrome, and have not tested this on any other browsers.
There is one more thing that I know temporarily fixes it, but has worked multiple times for me.
First, make sure that you have the sandbox for your course. If its not there just make it. As has been said on other posts, the courses usually require a specific project, such as the movies or the blank sandbox, so make the one you need.
Then, go to the home page for the course, the same page you were on when you first enrolled. Below the continue course button on the right side of the page, it shows the sandbox it uses for the course. Click on the link just below the title. It will bring you to a page where you can actually input the data you need. The first query should be ":server connect", and in its window it will have a few options.
Make sure the authentication type is 'Username/Password' then type in the username/password in the fields below. You can get the username and password from the connection details tab when you have the project selected, from your project list at sandbox.neo4j.com .
If when you run the query with that information, and return to the course, the query in the sandbox tab should start with "neo4j$" If it doesn't, try doing the excersises in the project from a separate browser tab. The project you did the ':server connect ' is the same one the course is trying to connect to, so you should be able to run your queries in there.
If you really want to have the sandbox tab in the course itself work, I have found changing the "Connect Url" drop down from "neo4j+s://" to the bolt option.
This has worked consistently for me whenever the courses start breaking.