I bought an ebook from kobo.com and authorized it by computer because kobo was not on the vendor list. I then tried transferring it to my ebook reader, which Abobe Digital Editions did successfully, but I couldn't open it on the reader because it was unauthorized.
At this point I contacted kobo.com support and authorized my computer and my reader using my Adobe ID, but when I tried getting the book onto the reader, I kept getting the E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER error. I also removed the .epub file from my reader. Next, I removed the ebook from ADE library on my PC to try and add it again, but this time I couldn't even add it to ADE library on my PC: E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER, again.
kobo.com support representative reset the DRM and I re-downloaded the .acsm file, but I still got E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER when I tried to add it to ADE library on my PC. At this point the support representative told me "If you are getting the same message then you will need to contact ADE support, I already reset the DRM so the book now is ready to be authorize with the new id".
Now, 7 hours later, I re-downloaded the .acsm file again in case it didn't reset instantly this morning, but I keep getting the same error.
I have seen the support page Solution for E_LIC error in Adobe Digital Editions but, obviously, it didn't help because the book's DRM had already been reset this morning.
So, what should I do so that I can read my book?
Seems like it was a vendor problem after all: I've contacted their support again and they did something (which took them about a week) that solved the issue.