E_ADEPT_DOCUMENT_TOO_SHORT

Hello,

I have trouble with an E-book I downloaded from my local library. I can download the .ascm-file but when I try to open it I get the error message E_ADEPT_DOCUMENT_TOO_SHORT. I deauthorized and reauthorized my ADE several times with no effect. I deleted and re-downloaded the files several times with no effect. I uninstalled and reinstalled my ADE several times with no effect. I contacted my library and after long E-mail back and forth I got the answer that it has to be a local error as the file is perfectly fine at the support guy's computer.

I tried to google a solution myself and found that "deleting the adept key in the registry editor" might be the solution: but what does that mean? How do I delete the adept key in the registry error?

Maybe also interesting: the file is somehow saved in cloud sync. I tried to delete the cloud sync file, which I can do, but when I reopen the ADE the book is (empty) in the library again...

Thanks for your help