Let's back up a little. I've been assuming you are getting a Crash Report, but I see now that may not be the case.
Where are you seeing this internal error? From calibre KFX plugin?
Perhaps it is an issue with the ePub files in question. Do they pass epubcheck? If not, I think it can generate 'internal error'. For another 'opinion', try opening in Apple Books. It's pretty picky as well.
If you have
brew (Mac package manager), you can install
epubcheck with
brew install epubcheck. There's a calibre plugin to run it.
if you don't have brew installed, here's how:
https://brew.sh