So, I just chatted with the Minerva chatbot and an agent who claimed to reset the license for the book and advised me to update ADE before trying again.
Since I'm using Linux, I don't want to risk breaking my currently working version of ADE 2.0 for an update that might not help.
Has anyone else had a license reset that solved a download error before? Should I try to download again with ADE 2.0, risk updating ADE, or potentially switch to the Calibre DeACSM plugin and give up ADE entirely?
*edit*
The error persists on ADE 2.0, even after deleting the Adept key from the registry and re-authorising yet again.