From your question what I infer is that after you have upgraded your server to SAS 9.4 M6, one user is having issues connecting to the server from a particular machine. The same user can connect from other machines.
If the user can connect from other machines, then the issue is not with SAS Server or upgrade. It is local to your machine.
I would suggest the following
1. The Metadata Server logs may give an idea in case of failed logins. Did you get a chance to look at the logs? This is the first place an admin looks in case of login failure.
2.If there is no entry for the failed attempt then please check network connectivity.
If the ping is unsuccessful then network is an issue. Resolve here.
If the ping is successful, then go to the next step (network not an issue).
3.Reboot the machine from where the user is not able to login..
4.If that does not solve the issue, rebuild the user profile on the Desktop.
5.If that does not solve then reinstall may be a solution.
May be you may resolved the issue by now. If then please share your experience.