This error (in any list) is almost always caused by an error in the data itself. In this case perhaps the use of an “odd” character in a user or real name. Perhaps an accidental carriage return?
Occasionally it is a timing issue between server and client, but that will usually disappear on re-boot.
If you know how to use firebug (or its equivalent in browsers other than Firefox) you can see exactly what the offending data element is.
Beyond that it is impossible to diagnose without full administrative access to your site.