It seems there's an underlying conflict with MS OS and Adobe Digital Editions 4.5.9., i.e., Windows seems to disregard the ADE.exe file after upgrading to ADE 4.5.9., as it won't display it as an available application in the File Association window even though it lists it in the Default Apps list. There's loads of traffic on the internet about this and nobody seems to have found a solution yet - Adobe are markedly silent on this topic! But I discovered a workround that worked for me and I thought I'd share it to see others find it helpful. Note: I'm using Windows 10, so can't comment on whether this work round will work on other Window versions.
- Click start on the main screen
- in Search box enter "*.acsm* without the quotes
- right click on any .acsm file and select "Open with"
- in "Open with" window select ADE and tick the box "Always open with" and click "OK".
- ADE should now open and the book should appear
- Go to your online library and download an ebook
- ADE should now open and fulfil the book.
Finished!"
Hope this helps.