I've been working on trying to get a customer import working and whenever I tried to run the import the Ajax spinner would just spin and spin. Looking at the console log I noticed that it was returning an error 500 in validation.js. After way too much investigation I was able to narrow this down to line break issues within the the import file. When I edited the file in vi it was showing a ^M character as the line break. I did a s/^M/^M and for some reason that fixed it. If you edited your CSV in Excel at all, even if you then copied that text into another app and did a copy/paste to try and strip the extra characters you might still have issues. It looks like if I export from Google Sheets it's fine.
I'm just putting this out there in case someone else is having similar issues.
It would be better to report in the issue tracker: https://github.com/magento/magento2/issues