Problem: License key is damaged ?

Just down loaded Adobe Digital Editions (ADE) and bought a digital book  from bokus.se  (first time) (Sweden). I tried to load / read the book and it was ok. When I opened ADE a second time (some hours later) I got an error : Permissions -  License key is damaged . I'm using a PC with Windows 7.  I also down loaded the same version on my Surface Pro 2 ( win8.1) it works fine there.  Question Where do I find my license key?  and what can I do to correct it?