I have the same problem; Schwab Street Smart Edge will not run on my newer computer, although it used to run for a long time. It ran in Windows 8.1. It stopped running for no apparent reason. Only system changes were program updates such as Windows as far as I know. An invalid log in message is displayed when log in info is entered and log in is attempted. Schwab error 204 is generated. I upgraded to Windows 10 and SSE will still not run.
I spent a long time with Schwab tech and he concluded that the log in request never gets to Schwab servers. So there was nothing more he could do.
I attempted the clean boot as you describe here and got no improvement, running normally or as Administrator.
I have an older, backup computer running Windows 10 and previously Windows 8.1. Street Smart Edge runs fine on this system.
This is a real problem and I am hoping you can help.
BB77SLR