I have a similar problem. I just uploaded the xml file of an old WordPress website, it's about 1 MB so not even close to the maximum of 8 MB. But it takes for ever to upload. After almost half an hour the system is still loading... And I can't find any time indicator whatsoever.
I noticed that the latest version of the plugin is from October, but the newest WordPress version is more recent. So... might that be causing the plugin to fail?
Please update the plugin or do whatever is needed to debug - I really need the old blogposts for my client to work.
Have a nice day.