Hey guys ... just got back from vacation a day or so ago and I'm just getting around to catching up on some online stuff.
Anywho ... yes, this plugin is still being actively maintained and updated. I'm a little behind the times with staying in sync with the underlying stand-alone mobi_unpack engine, but I don't think the latest update(s) in the stand-alone code would change anything here.
Dwanthy asked some excellent questions. I would have asked them all—and the answers to all of them will be very helpful in debugging. In addition, to those questions, I would ask: is that ebook file viewable with calibre (or other mobi/kindle devices/apps) before unpacking? Because it sure looks/sounds like a corrupt/unexpected/unprecedented MOBI header.
The error is occurring in the underlying mobi_unpack code, so if you have the ability to run python scripts, you may want to try using the very latest stand-alone script (link in the first post in this thread) and see if it produces the same error. Enabling debug mode will provide even more detailed output that may help identify the issue.
The very best way to troubleshoot this would be if I had a free, legal copy of the ebook file in question. If it's a retail ebook and it's free or very, very cheap, feel free to give me the details on how I can lay my hands on it (or one that exhibits the same behavior). Just don't upload the file here if there are ANY questions about copyright/legality. Feel free to PM me if you have any copyright concerns.