The issue seems rather complicated and probably relates to document permissions.
I have uploaded all my documents including my purchased Ebooks onto Document Cloud as I prefer the feature of document portability rather than carrying always with me the same device with the documents and My Editions software installed.
I can access my personal documents from any computer, but I cannot however access my purchased books online just by using Adobe Document Cloud, as the Adobe software asks for a password. I have registered and opened an account with Adobe My Editions exactly for this reason, i.e. to be able to access my documents with my password wherever I am. However, this system does not seem to work even after I using my Adobe My Editions login details.
It seems a very complicated technical issue to me and probably involves the publishers permissions too.
I do not expect anyone to have encountered anything similar. Please check the attached screenshot: I have a password for the documents shown but it does not seem to work within the Document Cloud of Adobe.