In order to fix this error your Activation needs to be reset which can only be done by Adobe.
As Adobe does not provides phone support for Digital Editions so you need to log a web case with Adobe there is no any other way to get this done.
To log in web case with Adobe visit following link : http://www.adobe.com/support/digitaleditions/
On the right side of the page click on : Submit a web case
Once you submit web case they will reset Activation with in 24-48 Hrs
Please Note : While submitting the web case do not forget to mention your e-mail address and the exact error message that you are getting.
I have also had this error twice now. You can also start a chat session and they
can do it immediately while in your chat session. I have opened a support case becuase I am confused why this is happening. I have to activate my computer each time I open ADE? That doesn't make sense.