I experienced a similar error.
The trouble is that loading-page.html has a incorrect address localhost:8080 for jupyter notebook. The connection to Jupyter notebook is localhost:8888/ not localhost:8080 now.
I assume your started Mac Application of Sagemath by Double click, Dock or LaunchPad.
Please verify your opening window has heading:
"The SageNB server or Jupyter is currently starting. Please wait..."
There are buttons "SageNB interface" and "Jupyter". Both of them link to localhost:8080 mistakenly.
Please try opening localhost:8888 form your browser.
Or if you are familiar with Terminal.app, please use the command
If this opens a Jupyter notebook or its login page, my workaround should be good for you.
My workaround is to edit loading-page.html by a text editor.
You should login MacOS with the same account as the time of your installation of SageMath.
If you put
SageMath-9.0.app in /Applications folder, you can locate the loading-page.html by the following pathname.
If the version is different change "9.0" with the version of your SageMath application.
(Form the Finder, the filename might appear SageMath-9.0 but from the Terminal, we have to write
SageMath-9.0.app with the extension .app.)
Opening this file from the Finder needs a technique of typing the path /Applications/.../Resources/ in the Finder's menu of "Go".
But it is simpler to use /Applications/Utilities/Terminal.app and open the pathname /Applications/.../loading-page.html by your preferred text editor.
Once opened loading-page.html, search for the string 8080.
The second hit is in the following patter.
<a href="http://localhost:8080/">Jupyter</a>
Change this occurrence of 8080 to 8888.
Please tell me if it succeeds.
I am planning to tell the developers many many improvements like this.
If some developer is reading my comment, please arrange you I can report bugs and improvements.
2020年2月23日日曜日 4時09分44秒 UTC+9 James Traynor: