Normally this error implies a session or cookie issue.
Please try signing out of your bookshelf/account then clearing your browser cache and then sign back in and retry opening the Online Reader.
Information on how to clear your cache can be found here.
If this process does not work and you receive the AER59912 error again please contact our support team. Please quote the error AER59912, tell us which of your ebooks is experiencing the issue, and explain you already tried the solution above.