When visiting the User page through Setup there is an error stating "Cannot sort using unknown attribute" followed by a long series of numbers. This is preventing me from seeing the users, or searching for them. Generally, when this error is displayed, there are no columns or user information shown on the screen. Adding a column such as 'User Name" will resolve the problem.
When getting this error please click the "Show Columns" which appears as an icon with three horizontal lines and a plus symbol. From there locate the "User Name" entry and add it to the page. Once the column has been added click on its header so that the page will sort by it. After that the users should be displayed again.