John and Karen both use the same computer to clock in and out. John is able to clock in and out just fine, but Karen gets a message saying the TimeTracker has been deactivated.
The problem was that the public IP address of John and Karen's computer was not registered in Preferences > General Account Info. In this case, the cause of the disparity was that John had an extra permission enabled in his profile. He had the option to "allow user to submit time entries off-site." This means that John had the ability to clock in from any computer--at home or at work--no matter that his IP address was not listed in the approved IPs table.
To resolve this type of situation, the IP address first needs to be added to the Allowed IPs table. This is done by going to Preferences > General Account info. Once there, your current IP address is listed in red under "Allowed IPs" and is also copied into the next available cell. All you need to do is click submit to save the information.
Next, you would need to determine if the first user (John, in this case) is indeed allowed to clock in offsite. If not, remove that permission from his profile: go to Preferences > Personnel/Employees, click on his name in the list, and uncheck the box that says "allow user to submit time entries off-site." Submit the form to save the information.