[This also fixes to the "Add to Quick Access button permanently greyed out" issue]
As others found out, you have to delete the contents of the following folder:
This is a folder that won't appear in Windows Explorer regardless of whether having enabled the "Show Hidden Items" option, so you have to invoke a Command Prompt to deal with it.
Furthermore, you might find out that deleting the above directory contents does not work. You may see the following error:
The System Cannot Find The Path Specified
or another error similar to that.
Therefore, the step-by-step procedure to restore the functionality without errors is the following:
Open a Command Prompt (may require Administrator privileges).
Copy and paste the following command:
del /F /Q "%APPDATA%\Microsoft\Windows\Recent\AutomaticDestinations\*"
Notice the double quotes: they are required if your user name contains spaces. Otherwise, you get the "... cannot find the path specified" error. This happens because %APPDATA% would expand to space-separated multiple words and DOS interprets them as separate parameters.
- Press ENTER and wait.
After about two seconds you should see Quick access changing, often to completely different items. You'll see the "Freshly installed Windows" items plus others.
In my case, it restored some 5 years old quick access items (!!!). They would overlap and break the ability to add new items. I removed them and then the Quick Access action returned to full functionality.