N SVG version of the Finder icon from macOS Big Sur

You can modify the minimum thumbnail size in the info.plist (in .../DropboxQL.qlgenerator/Contents/):

<?xml version="1.0" encoding="UTF-8"?>
    ...
    <key>QLSupportsConcurrentRequests</key>
    <false/>
    <key>QLThumbnailMinimumSize</key>
    <real>17</real>
</dict>
</plist>

After editing in a smaller size in the plist and a reboot, you will get a preview for 16x16 pixel icons in the Finder again.

The DropboxQL.qlgenerator usually is located in /Applications/Dropbox.app/Contents/Resources/

Also check for older DropboxQL.qlgenerator in other places and either remove them or also modify the Info.plist.

The priority order from first to least seems to be:

• app embedded qlgenerators
• ~/Library/QuickLook/
• /Library/QuickLook/
• /System/Library/QuickLook/

Any problem that only occurs in one user account has a standard solution:

Your problem is caused by something in your user account. This could be:

• third-party software that is triggered to launch automatically, or something like a Finder extension.

• A setting or preference that is wrongly configured.

• Something else that exists inside your ~/Library folder. (the ~ sign means the current user's home)

Check System Preferences > Users & Groups > Login Items to see what is set to load at Login. It's possible that one of those might be incompatible with Big Sur and causing problems. Check for updates. Disable all of them, reboot and test the problem.

Check Finder Extensions in System Preferences > Extensions. Test the problem with any third-party extensions disabled. (You'll need to reboot or restart Finder, at least.)

Check the LaunchAgents folder in the ~/Library. This may contain scripts that trigger background apps. These may need updating. Again, remove the plist files to another location, reboot and test.

If none of those things work, then you'll need to 'divide and conquer'. Start removing entire folders from the ~/Library, reboot and test. If the problem is still there, replace the folder. If it goes away, restore half the contents of that folder, and test again. Gradually, you'll focus on the problem.

Likely folders are the Preferences folder, which contains ... well, preferences; and the Containers folder, which contains individual apps' working files.

Note that this is all in the user's Library, not the root level Library. The user's Library may be hidden in the Finder. Hold the Option key and click on the Go menu in the Finder, and Library should be a menu item.

Things not to do:

• Don't reinstall the OS. The OS is working fine, as shown in the new user account. That will do nothing, except waste your time.

• Don't migrate to the new user account. Changing user account can cause problems with existing file permissions. Also, if it happens again, will you keep moving on to new account after new account?