You may find yourself unable to log into the Web Client with your username and password after performing an Operating System upgrade on your FaxMaker server. You intend to upgrade FaxMaker as well but did the OS first.
The correct order for such scenarios is that you upgrade FaxMaker first, and then the Operating System.
You can find the steps for this in the Upgrading the Operating System on the FaxMaker Server article.
If the log-in issue persists, please check the article Unable to Log in to the Faxmaker Web Interface.