Originally Posted by Kek
After some Database tweaking, yes it was the "case-problem" that caused those dead-links.
In fact sometimes Calibre don't change the casing of the epub file even if you do it right in the program. I've checked the faulty books manually to be sure.
Whatever it was, it's now working.
You might explore the Quality Check PI: Fix: Check and Rename book Paths.
(I think this adjusts
the internal DB reference to be the current path case.)