[tomsem]

There is nothing wrong with the Colorsoft, it will have to do with the specific ePub file in question. I suspect it is a fixed layout ePub; it's also possible that it has other problems, e.g. doesn't pass epubcheck.

A good test is to open with Kindle Previewer. If it barfs, then it will emit an error report, maybe you can fix and try converting again.

But if it is fixed layout ePub (comic book or some such), STK will not convert it properly: it does not support fixed layout ePub and will munge it into reflow able AZW3, and poorly at that. Calibre does not support fixed layout ePub, and will not convert it to usable fixed layout AZW3.

It does not seem like rocket science to do such a conversion, but I've been unable to discover a tool that does it.

I would look at source code for KindleUnpack; it will do AZW3 fixed layout to fixed layout ePub, maybe someone could reverse the workflow to do the reverse operation.

If it consists only of page images (without positioned text & embedded fonts) then you could extract the images (there's a function in calibre-edit to do this) and then build AZW3 with Kindle Comic Converter, or build KFX with Kindle Create and KFX output plugin. Of course either of those needs to be side loaded.