"Request Failed, try again" for some users

Using the login page that is included with M2, I have a very small subset of users who can neither login or reset their password. Each attempt is met with "Request Failed, try again". I have disabled all other plugins while testing with one of these users with no success. I have checked my Apache error logs and see nothing related to M2 or the customer's IP address. The strange thing is that most (if not all) of the effected users are in the UK.

Error Process:
- User attempts to login at https://graffitisuite.com/account/ using valid username and password
- Login form begins working
- Login form then displays error "Request Failed, try again"

Plugins I disabled for testing:
- WordFence Security
- Google Captcha (reCAPTCHA) by BestWebSoft
- WP Super Cache

PHP.ini Modifications for Troubleshooting:
upload_max_filesize = 100M
post_max_size = 105M
memory_limit = 768M
file_uploads = On
max_execution_time = 500

I have been unable to replicate this on any of my machines and browsers, and had to replicate it on an effected user's system via TeamViewer. Is there some other way of tracking this down that I haven't seen? Under what conditions is this incredibly unhelpful error shown?

I have read all of the other threads here that I can find on this issue, with no luck.

Checking the JavaScript console I can see an error on these users' computers stating that there are multiple instances of the #_wpnonce field.