https://www.akeebabackup.com/documentation/troubleshooter/atwafissues.html
You may want to add to the top of the page that the user should check that their IP address has not been changed by their ISP. If only one IP address is in the whitelist then when the ISP changes the user's IP address, they are no longer able to log into the administrator page. I have now set my whitelist IP address as:
xxx.xxx.xxx.
This will usually pickup the new IP address that is assigned to my Internet account.