Unable to download bought books due to "License already fulfilled by another user"

Dear all, 

all my life I have used the same Adobe ID. I have bought several ebooks on authorized online platforms back in 2020. I have been trying for hours to download said ebooks again and load them onto my new Tolino e-reader. It's been a real pain, nothing gets recognized, I get an error saying that the ebooks are matched with another Adobe ID (which can't be, I only have this one ID - never created another). I've tried everything from de-authorizing my PC to doing the same on my reader but nothing works. 

Please help me. I have about 10 unread books that I have bought and cannot use... please note that these are not borrowed or library books. I have bought them on official German bookseller websites, they are mine. 

All the best and thanks in advance.