You need to post what error you get when saving to disk. And you need to post an ebook file that does get get the current date when adding to calibre.
Generally speaking, if you are getting errors when doing a large batch and not when doing smaller batches, something on your system is breaking under the load. What that might be is anyone's guess.
Notice to all: I can not
provide assistance with DRM removal, for legal reasons, so please do not contact me about it.