If you know how to debug web stuff, check whether the response has the Strict-Transport-Security header.
If it does, the website asked the browser not to allow the user to see it if there is any security problem.
(Though the message usually tells that to the user, so it might not be it)
Adding Ryan (it would be great if you added someone else if this is not your area) -
While I appreciate the lengthy description in the screenshot, it would have been better if at least technical users could understand the exact problem (or something close to it), because using the word "credentials" is about as vague as one can get. ;)