I never investigated this deeper but I have similar issue on my Nook Glowlight. Despite it supporting ePUB files some of them just throw an error on the devices. Converting them ePUB to ePUB solves the problem.
I've learned recently at my new job that ePubs are just a giant zip file of html, css, etc. Sometimes you need to rezip them if they are not working quite right and I imagine that is what converting them from ePub to ePub is doing.
