Ah ha! Finally someone else with the same error.

I've got another update on my own situation. I had someone else on a completely different computer and Internet connection try to upload a CSV file on one of my stores, and he got the same error message, so that eliminates the possibility that it had something to do with my Internet connection or anything else locally. He suggested that perhaps some program file was corrupted during an upload or download. It couldn't be one in this add-on since the issue occurred with both versions I tried, so it would have to be one of the original ZC files, perhaps one that controls file uploads. However, now that someone else is also having the same problem, I don't see how it could be that since the odds of someone else having the same file corruption is pretty remote.