Hi, I have just been dealing with a similar issue, and I believe it is because my hosting provider, hostmetro, somehow injects that math problem CAPTCHA for WordPress logins. However, recently it has not been working with the Google Chrome browser. The hostmetro support person told me that they are aware of the Chrome issue and suggested some Firefox CAPTCHA plugin for Chrome as a workaround. I have not tried this workaround.
Hopefully somebody at Google is aware of this issue and is actively working on a solution as my Chromebook is my primary laptop for maintaining my WordPress site.
