Error message: E_AUTH_BAD_DEVICE_KEY_OR_PJCS12

[GizmoJones]

I have been using Adobe Digital Editions for several years.  Today, when I tried to add a book to my library, I received the error message:

Unable to download

Error getting License. License Server Communication Problem:

E_AUTH_BAD_DEVICE_KEY_OR_PJCS12

I checked my computer authorization, which tells me that this computer is already authorized. 

I have been downloading books from this particular source for several years with no problem.

What can I do to remedy this, please, without losing the books already in my library?

I am running Adobe Digital Editions 2.0.67275 on a Mac OS 10.10.4 (Yosemite).