[Karellen]

Recently Calibre has been returning an error in Check book when I copy a xhtml page from another book to the current book.

I am having trouble figuring out what the problem is.

In image 1, the case seems to be the same.
In image 2, the folder case is different, but when I change it, the error persists.


The fix is quite easy by running the "Arrange into folders" option, but I would like to know where to find the problem. This has only recently started being an issue, maybe some time in v6.9 or v6.10?
.