Yes, I did pay for this version. I did it because I thought it was going to unlock all of the different themes. Sadly, those themes seem to be an additional charge as, when I click on one, it's asks for a small tip to enable the color schemes. If you want to have access to all of them, it's pretty much the same cost as paying for the full version,.
Any way.....
When I switched to Marvin 3, I think I did the backup of my books in Classic and moving them to Marvin 3. It's been a while but I don't remember why I didn't keep all of those books in it or if, possibly it caused an issue similar to yours but I was in the process of prepping for a vacation and wound up just loading the ebooks I wanted to take with me and read from that point onward.
Wish I could be more help on this.