After much searching & struggling, found my own solution. Posting it to help others.
Running Windows 11
Syncthing had been starting automatically from Windows Task Scheduler. A beautiful, trouble-free setup.
After upgrading Syncthing to 2.0.5 (from a 1.3.4), it wouldn’t start.
Sometimes the Web GUI would demand authentication, but that ended up being irrelevant, not the issue.
Syncthing would start from the command prompt, but then stop when I closed the command prompt.
So when I ran “syncthing.exe -no-console -no-browser”, the problem was replicated.
But when I changed it to “syncthing.exe --no-console --no-browser”, PROBLEM SOLVED.
I made the change in my Windows Task Scheduler, and operations were back to normal.
Hope this helps someone.