Earlier this week, I was trying to fix that problem on the OSX computer of a colleague. It turnout that opening the jupyter notebook from the command line was working when using firefox instead of safari. For people not afraid of the terminal, the solution was to create a file .bash_profile in the home directory with the following content:
export PATH=/Applications/SageMath-9.0.app:\$PATH

Then running sagejupyter in the terminal in any chosen directory does the trick.