Thanks for all the suggestions.
Yes, Calibre is a 32 bit application. But under Windows 64 bit, each 32 bit application gets its own 32 bit address space, so it is very unlikely indeed to run out of memory. Far less likely than on a 32 bit operating system.
I have only 155 books in my Calibre collection.
I have only one other plugin. It is a popular file type plugin, let's say no more. I have never had any other plugin installed apart from earlier versions of the unpack one, including the last beta.
I just rebooted my computer, although this does not seem a logical thing to do. Result: it just unpacked it with no problems at all. And I carried on unpacking the file six times in all with no problem at all.
Nice to know that when desperate to Mobiunpack I can reboot.
But something is wrong somewhere.