This only applies to Opera. It's been an issue since as long as I can remember. I save HTML files on my computer, all the time. Like I wanted a copy of DavesComputerTips newsletter. So in my Seamonkey email program I just right click on a email and click on Save As... and pick HTML as the type.
This will save the latest email as a HTML file on my system named Dave's Computer Tips Weekly Recap Issue #278.html. So I double click on it to open it and I get the following error message from Opera:
Your file was not found
It may have been moved or deleted.
If I right click and use Open with option it still will not work with Opera but will work with every other browser on my system. Go figure. But I still have two options that do work. I discovered long ago that it was the pound key or number symbol # that was the culprit.
So, Option 1.
Rename the file and remove the pound symbol.
I can still open it if I open Opera and drag the HTML file from File explorer to the browser.
It's just a minor issue but after all these years I'm surprised that it's still an issue at all
Just a interesting final note. If I rename the HTML file and put the pound symbol in front of the name so it reads: #Dave's Computer Tips Weekly Recap Issue 278.html, it still will not open the HTML file but it will give me an explorer type window that allows me to see and navigate the files that are in the the same folder as the HTML one is.