Aura Download Epubs from Dropbox

I have my library on Dropbox and when I need a title for my Kobo Aura I simply connect it through wifi to my Dropbox account and can select and download the book I want.
But without having changed any settings it now suddenly looks and behaves as 'greyed-out': I can see my directories but tapping will not open them as before.

Anybody with the same experience, a solution and/or link thereto?

Thanks