In this new installation it works.
But, I tried again in the original installation and it now works there too. I have not changed anything. After more than a dozen failures I have only reported Operas problem and continued my reading and downloading of more then 25 story chapters using EDGE, while leaving Opera active but unused except for looking for answers here in the forum.
Did my original Opera installation just need some days of rest? I'm baffled.
I regret that I didn't check with the original installation before I installed the test version.
I would really like to know why it happened, I've an uneasy feeling about problems just going away.

When saving a web page to disk I can select from 3 file type choices (translated from German to English):
web page, HTML only
web page, single file
web page, complete
The "HTML only" is the culprit. It's the one I always (for years) used, so it shows up as default. The other two work without problems!
The drive and folder do not affect the problem, I use different folders for the story chapters of different authors and for a test I selected another drive (USB disk) too.
I'm now no longer certain the test installation really worked, it may have defaulted to "web page, complete" and I didn't see it. The resulting file name is the same and I hadn't seen the subfolder it created, because it was out of view at the start of the listing.
So now I've to do the test installation again and this time check if it really works with "Web page, HTML only".

When saving a web page to disk I can select from 3 file type choices (translated from German to English):
web page, HTML only
web page, single file
web page, complete
The "HTML only" is the culprit. It's the one I always (for years) used, so it shows up as default. The other two work without problems!
The drive and folder do not affect the problem, I use different folders for the story chapters of different authors and for a test I selected another drive (USB disk) too.
I'm now no longer certain the test installation really worked, it may have defaulted to "web page, complete" and I didn't see it. The resulting file name is the same and I hadn't seen the subfolder it created, because it was out of view at the start of the listing.
So now I've to do the test installation again and this time check if it really works with "Web page, HTML only".

I deleted the test installation and now the old installation stopped saving to disk again. Arrggg!

In this new installation it works.
But, I tried again in the original installation and it now works there too. I have not changed anything. After more than a dozen failures I have only reported Operas problem and continued my reading and downloading of more then 25 story chapters using EDGE, while leaving Opera active but unused except for looking for answers here in the forum.
Did my original Opera installation just need some days of rest? I'm baffled.
I regret that I didn't check with the original installation before I installed the test version.
I would really like to know why it happened, I've an uneasy feeling about problems just going away.

Starting after 2020-10-09 00:32 (local time of last successful download) Opera reports "download complete" but when I click on the folder symbol Opera can't find the file. Using Explorer confirms the file is missing.
I updated Opera and then Windows 10 Home, but saving a page as a html file still doesn't work.

For test purposes I tried the very same pages in Edge and they got stored flawlessly!

Seems to me that Opera does not get any error back from the OS, but the data get lost somewhere in the process or deliberately discarded by Windows.