Please ignore my post. I actually found a response from an Adobe support team on another posting thread for the same topic. I just copied his/her answer here in case some of you are interested. When I followed the following instruction, it fixed my issue.
For this problem, you need to deauthorize and authorize ADE again.
Help -> Erase authorization.
ADE will be deauhorized.
Now again AUTHORIZE ADE
Help-> authorize computer.
Error will be fixed !!
If due to some reason, you are unable to deauthorize ADE [error to try after some time], follow the below steps:
1. Go-> Go to folder.
Go to folder dialog will appear.
2. Type ~/Library/Application Support/Adobe/Digital Editions
drag the activation.dat file to the trash.
ADE will be deauthorized now.
Now Authorize ADE again.
Choose Start > Run.
In the Open text box, type regedit and then press Enter. The Registry Editor opens.
In the left pane of the Registry Editor, locate the following registry key: