The issue you have been experiencing is to do with special characters within the file name causing issues for you ereader device (e.g. the%20hobbit.epub).
The answer is to rename the file within ADE before transferring to your device.
If you open ADE on your computer and go to the Library view. Then right click on the book cover (control click for Mac) of the book and select 'Show File in Explorer' ("Show File in Finder" for Mac).
If you remove any special characters like full stops, apostrophes, commas or anything like that (Examples: %#&-+;!) so you are left with a file with a fairly simple file name.
Changing the name will make ADE unable to find the book, so in ADE you need to go to 'File > Add to Library' then navigate to the new file, select it and press 'Open'. This version of the file should copy across to your device and open without issue.
The old version of the file will still show in your ADE library with a little note across the cover saying 'missing'. You can right click (control-click for Mac) on the cover and select 'Remove from Library' to get rid of it.
If this does not resolve the issue please contact our support team. Please quote the error message when doing so, and include the filename of your ebook.