Trouble accessing Samsung Knox & resetting the password
Ok, I seem to have resolved this!

I restarted the device while it was on the Knox lock screen, and it booted up normally.

Going back in to Knox asked me to set a password (this may or may not have been because of the steps I followed on the website as described in the question above).

Either way, it appears that either updating Android or getting stuck logging in to Knox through the Google Play Store notification was the cause of the issue.

Restarting the device again and attempting to access Knox now lets me in.

(I'll leave this here in case anyone else comes across the same issue!)