The problem is that PDF24 creates a new user to access the folder with temporary files. And this user does not have rights to write or change data.
Solution - run PDF24 as administrator.
Also, as an alternative, you can give rights to write and change data to the user created by PDF24 (by going to this folder - clicking "properties" - then go to the "security" tab - "advanced" - "change rights").
I have tried both methods and both methods work.
Good luck!