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. 

E_ADEPT_REQUEST_EXPIRED
You can sometimes get the following error message while activating Adobe Digital Editions:
E_ADEPT_REQUEST_EXPIRED http://adeactivate.adobe.com/adept/Activate
If any of the following system settings are incorrect, this error can occur: You can also get the error if you don't open the file downloaded from Ebook Central in a reasonable amount of time.  The .acsm files downloaded from Ebook Central have an expiration time (set by Adobe) and if you don't open the .acsm file within about 15 minutes of downloading it the title won't open in Adobe Digital Editions.

If you receive this error please check your computer's time, timezone and daylight savings settings and then download and then immediately open the file with Adobe Digital Editions.

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