Basically when the scanner warms up, he shows a preview image of what's on the scanner device. However upon pressing the scan button you get the message "The selected folder is not writable". The first reaction is to look at the destination folder... wrong guess.
If you change your view by clicking the "Show Details" button you will see a bit more options. Change the Image Correction to "Manual" and that's it. You don't need to change anything else. Now you should be able to scan your page.
Clearly a software bug, so @canon, if you read this; fix it please :)