500 Server Error Jupyter Notebooks on Ubuntu 20.04

Hi, newbie here, I’m working on an installation of RStudio. At some point Jupyter Studio started throwing 500 errors even though it was working before. I have a another environment where it’s working. For comparison:
Environment 1 (good): Serving notebooks from local directory: /home/admin, running on :8888
Environment 2 (500 error): Serving notebooks from local directory: /usr/local/share/jupyter, :8888 and :8889 are already in use, running on 8890
I’m not sure what to check here, or what’s a problem and what is red herring.
Help appreciated!
Thanks,
Sco