This usually happens when the user has been deleted, so it no longer can show user details at the opportunity list.

You can check with a simple query in the SQL Console

Select user_deleted, * from users where user_userid = 83

If the value of the column user_deleted is not null, the user has been deleted.

Hope this helps.

If they have been deleted, undelete them and instead mark them as Disabled - this will hide their name from any user select list, not allow them to log in, frees up a licence (if a named licence system). But keeps their name for historic purposes like your screenshot above.

The user is not deleted, any other ideas?

Hi Zach 

Have you tried a Metadata refresh? (Admin | System | Metadata Refresh) 

I just tried refreshing the metadata again, unfortunately it still looks the same.

Look at the properties on the list. Check if the column is marked as "Show as Gif"

Tino,

You got it!  Thank you so much for your help!  It was marked to show as Gif, when I turned show as Gif off it is now displaying the user name.

Thanks to all for your help!