Most common Adobe error messages experienced on Ebook Central are listed below:

E_ADEPT_NOT_READY or E_ACT_NOT_READY
If you are receiving the Adobe Digital Editions error "E_ADEPT_NOT_READY” or "E_ACT_NOT_READY" when downloading an ebook, according to Adobe, this error occurs when the authorization information is damaged. The solution is to rebuilding the activation file.  For the step by step instructions on how to do this (de-auth and re-auth ADE), please refer to our support article available at: https://support.proquest.com/#articledetail?id=kA140000000Th1nCAC

E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER
According to Adobe, this error message indicates that you are trying to open a book that was already downloaded to another Adobe ID under the current download period.
Please refer to our support article for more info on this, available at: https://support.proquest.com/articledetail?id=kA01W000000TVq8SAG

E_ADEPT_NO_TOKEN
If you are receiving the Adobe Digital Editions error “Unable to download. Error getting License. License Server Communication Problem: "E_ADEPT_NO_TOKEN” when downloading an ebook from Ebook Central, contact Ebook Technical Support (ebooksupport@proquest.com, or Submit a Case) to report the error.  Please provide Technical Support with the Ebook Central URL for the ebook, the title of the ebook (including the Document ID or ISBN), and a screen shot of the error message. 

Other Adobe Digital Editions errors might be resolved by de-authorizing and re-authorizing Adobe Digital Editions. 

In addition please refer to Adobe's Website for more information with regards to Activation Errors at:
https://helpx.adobe.com/digital-editions/kb/activation-errors-adobe-digital-editions.html