I happily got an answer to this today from Canon telephone Technical support-- two techs involving two calls, both of which were friendly and helpful.
The first one gave me a symptom solution which at least got my scanner utility working (go to scanner SETTINGS on the computer app and re-direct the SAVE file location for each individual function.) Time consuming and a hassle, but it worked, mostly.
But then I thought about it and wondered, "Yeah, but what's the disease, the core-root problem?" The second tech answered that, and helped me fix it.
As I suspected it's primarily a WINDOWS change from back in 2014 which messed up access to specific folders, such as DOCUMENTS. It's part of the security system and it basically says, "Windows isn't going to allow access to this folder any more by any application." I don't know why my other applications got through and the CanonScan didn't... and I don't know why the problem didn't affect me until recently (most likely Microsoft made yet another change in 2020 that really threw sand in the gears). But the tech helped me fix the problem. Here is the solution:
START BUTTON / SETTINGS / UPDATE & SECURITY
WINDOWS DEFENDER or WINDOWS SECURITY (your option may be different. Look for a shield icon.)
Look for an option that relates to app and folder protection. The heading changes depending on what version of Windows you are using. Even Windows 10 users will have different menu options. Chances are though your desired option will be:
RANSOMWARE PROTECTION
ALLOW APP THROUGH CONTROLLED FOLDER ACCESS
ADD AN ALLOWED APP
RECENTLY BLOCKED APPS
If your app shows up, click it. That will allow the app to access the folder. If not, you can ADD a specific app.
Alternately, if you don't want a folder protected (such as DOCUMENTS... which hasn't been protected prior) instead choose the RANSOMWARE PROTECTION / PROTECTED FOLDERS option, and remove that folder from the protected list.
These steps are a hassle of course... but it should fix this problem. Hope it does for you.
Note: because of inconsistency in Windows versions it may be a little tricky to find the exact settings I've described above. You may have to do a little hunting and logical deduction to find the settings described.
