If you are having login issues you will likely find your way to this thread, below are a number of things that could cause login issues along with an easy fix for the specific issue.
All of these issues are caused by the login request being tampered with or blocked entirely so the issue will always be related to something (setting, software or hardware) related to affecting the internet connection or software.
Problem 1 - Bad proxy being set in Windows Connection Settings (Most Common Issue)
Solution - Remove the proxy from your Windows Connection Settings
Problem 2 - Anti-Virus or Security Software
Solution - Set an exception for the software or disable it while your SEO software is running. None of my software allows any potentially dangerous code to run on your machine so it is safe to run any of my software without an AV or security software. The exact method for setting an exception or disabling will very much depend on the software being used, in this case Google is your friend. Alternatively upgrade to a less interfering anti-virus software.
Problem 3 - Windows .hosts redirect
Solution - If this is the fault I am sure you know why it is there and how to remove it but in case it was not entered by yourself you can follow this guide (Win7 / 8 / 8.1 for earlier version of Windows follow the guide for win7) to get to your .hosts file. The entry you want to look for is relating to bhwhtools.net, if there is any mention of this domain remove it and you should be good to go.
Problem 4 - VPN Software
Solution - This is similar to a bad proxy only it should be as simple as disabling the software
Problem 5 - Your Router
Solution - *Shrugs* Try a different router? With so many different makes and models of router it is impossible to say if that is the cause or how to fix it. I should say it is unlikely this will be the cause but it is a possibility.