The firmware version you uploaded via Ymodem has to match the version on the scope, or be close to it.
Secondly, if i am not mistaken, ability to boot from USB was removed at some point. I believe this is what TheSteve referred to.
I know that some people have had to try several (older) versions to find a matching one. I
suspect that this was just due to an error in the original guide (wrong .lnk startup file) that meant the application software was loaded from NAND and not USB as intended. But I cannot be sure.
IIRC version 2.35 was indeed the last one that supported running from USB. Later versions will instead halt the boot process and print the IP address (if the network module is connected). But this is baked into the system image, so if you load 2.35 (or older) through YMODEM, it should work not matter what version is installed.
In any case, the log clearly shows that the oscilloscope is in fact trying to load from USB, it just doesn't work for some reason. If you have double-checked that the contents of the startup .lnk file and the Secure folder are correct, trying different versions and maybe a different USB stick might still be worth a try.