This has plagued me for the last week. I have tried everything I can think of, and everything that I've managed to find by googling the symptoms, but the issue only goes away for short amounts of time.
Summary: The issue is that some users will open their browser, which directs them to their homepage: http://sharepoint (our company intranet website, running on SharePoint 2010 on MSServer2k8) and they will be greeted by an empty credentials prompt. Entering their credentials results in it remembering the username, but prompting for the password again. It repeats this until it fails with the message "401 unauthorised" and a blank page. You can sort of get around the issue by using our FQDN - http://sharepoint.domain.local - you still get the password prompt, but you can click cancel and it will go away and still load the page. This reoccurs when you click any link, however, so it is not a good solution.
I initially thought it was due to a bad password / old cached password, but that is definitely not the case. I have used AD to reset their password, as well as having them reset their password via ctrl + alt + del. I have emptied Internet Explorer's cache, as well as Chrome's, and Windows Credential Manager. I have seen a few registry edit suggestions for people in similar situations, but they did not work. Neither did removing the foxit reader add-in (well, it did, but only for 24 hours).
I'm totally stumped. This is only affecting three users. When it is affecting them, they cannot log on to SharePoint via ANY PC. Also, I can still log on to SharePoint on their machines no problem.
It seems to happen around 9-9:30AM and sometimes in the afternoon around 3:30PM. There have been no changes to our SharePoint server in the past week.
Really hope someone can help!