Spyder flashes "Initializing" then dies

Question

After installing Spyder using WinPython, Spyder opened once and worked properly for a session. Since then, when I open it, the opening screen flashes, the word Initializing... comes up, then the program closes.

I've shut off the Antivirus and Firewall, restarted the machine, tried reinstalling WinPython. Nothing works.

What happens in that Initializing sequence that might fail?

OS: Windows 11, up-to-date.