I've run into this problem again today, and discovered a solution quite by accident. (The previous times I've encountered the problem, it eventually corrected itself.)
Because I wanted to bulk-relist a large number of books, I had to switch over to the "seller hub," since the ability to bulk-relist has disappeared from the previous version of the page. I don't know if the solution I'm about to describe works when not using the "seller hub."
Under "manage unsold listings," click on the little drop-down arrow that's between "sell similar" and the photo of the item. Then click "relist."
Once I clicked "relist" on one of the books that had suddenly developed a "category" issue, I discovered when the listing came up for editing that the screen has an entire section that the other listings (the ones that didn't have category problems) don't have. And there's a check mark in a box that says "Prefill this listing with eBay catalog info." Once I unchecked the box, the category problem went away. On a couple of the listings I had to add back in a description and a photo, but then they relisted just fine, complete with the category that was previously alleged to be unacceptable.
All of the listings that I had to perform this "surgery" on had been listed multiple times previously. Why eBay suddenly decides to "choke" on any given listing is still a mystery... but hopefully this solution (that I discovered by accident) will be helpful for those of you who are having the same issue. Best of luck!
