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Update: I can get to the Edit User detail page by substituting the url-encoded plus character in the hash portion of the url:
https://localhost:44301/EPiServer/EPiServer.Cms.UI.Admin/default#/AccessRights/Users/UserDetail/dddouglas%2btestreg5828@brillianceweb.com
Duplicate of
https://world.optimizely.com/forum/developer-forum/cms-12/thread-container/2022/3/admin-ui-bug-user-detail-404-in-cms-admin-when-username-contains-plus-character-/
Clicking on a user name with a "+" in it in the Admin UI results in an empty Edit Detail page for the user.
The problem is that the userName parameter is not URL encoded before sending. Therefore, the controller receives a space character (" ") instead of a plus character ("+") in the action's parameter. Because the unescaped username does not match the user's name in the database, the user is not found and the blank detail page is presented.