@antoine-luboz I did just that. I opened it and tried to see what could have posed a problem. I ran it through a validator, and fixed the 1 issue it found, and the mht still wouldn't load.
I also can't see any errors being given as it reads the file. It's all very strange.
@Gwen-Dragon Just to be perfectly clear: what I did was enter the entire path to the executable, i.e. in this case C:\Users\Harry\AppData\Local\Vivaldi\Application\vivaldi.exe
Incidentally I have v. 1803, OS build 17134.590