Error E_ADEPT_INTERNAL

I have a Kobo .acsm book that has worked in the past. Now ADE 4.5.1 shows the error:

Unable to download

Error getting License. License Server Communication Problem: E_ADEPT_INTERNAL

I have, of course, tried this: Error "E_ADEPT_INTERNAL" | Download protected e-book | Digital Editions on the computer that had successfully opened this book before, as well as tried installing ADE from scratch on a different computer where ADE had never been installed before, downloaded the .acsm and obtained the same error. I can confirm that the said article does not solve the problem.

How do I open my book?