I know I am not the first person to have an issue with this. The most recent question I found regarding this issue is from 2014 and it was not answered (see: content locked message on Kobo )
I have been downloading books from my library and reading them on my Kobo by using Adobe Digital Editions to copy. All of sudden, anything I try to download and read gives me the following message: "Content is locked. Please use your Adobe ID to authorize your eReader with the Kobo Desktop Applicaiton or Adobe Digital Editions".
I had been previously provide support from Adobe on this issue. They provided me with the steps to deauthorize and then reauthorize.
I followed these steps again this time (twice!) and the issue remains unresolved.