Agile Central - Administration: "Cannot sort using unknown attribute" Error on the User Page

Document ID : KB000095647
Last Modified Date : 14/05/2018
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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.

                                                              User-added image   
A column that the page is sorting on (or needs in order to render, such as User Name) has somehow been removed from the grid, so the page cannot display properly. 
Add the missing column back into the page. In most cases, this is the "User Name" column but could certainly be any column/field that the page was set to sort on.
  1. Please click the "Show Columns" icon with three horizontal lines and a plus symbol
  2. From there, locate the "User Name" entry and add it to the page
User-added image

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.