OK, thanks for letting me know.
This is so odd. We’ve been running CryoSPARC for years with no issues until this OS reboot (we didn’t even update CryoSPARC!) and yet SSH tunneling as described did resolve the issue - the UI now loads and I can login.
Thanks so much for the help. I’ll chat with our IT folks and see if we can come up with a more permanent solution. Sounds like reverse proxy is the way to go?