kazbates:
The major difference between epub2 and epub3 (for EpubSplit) is that the Table of Contents is in different files using different formats.
The previous version of EpubSplit could handle epub2 and some epub3
also include epub2 Table of Contents(TOC). This is a thing that some epub3s include specifically for backward compatibility.
However, Calibre doesn't normally include that older/back-compatible TOC file when writing or converting epub3. Those just failed in EpubSplit with a non-descriptive error. (See back a few posts.)
Calibre's output is the only source of epub3 examples I've yet found, and they all use the same code. That's not a large enough sample for me to add epub3 code to EpubSplit yet.
I did consider looking for existence of the older TOC file, but there's no guarantee in back-compatible epub3s that both TOCs are the same.
So your choices are to down-convert your epub3s to epub2--Calibre Convert's epub output has that option; or, as you asked, use an older version.
Past versions of EpubSplit are available here:
https://github.com/JimmXinu/EpubSplit/releases