@n080dy, yes, I did work it out in the end.
Short version is that while I was going through the restore process, my browser kept suggesting a password. Even though I didn’t use it, at some stage I was force to hit the “back” arrow in the browser to get through things. Umbrel did not like this, and was causing the error.
I opened the Umbrel service from another machine without the password suggestion, which allowed me to linearly go through the workflow and it worked fine.