This error occurs in the following situation:
Sorting on owner/changedBy works correctly if you show the supertype in the datagrid, but not if you show a type where owner/changedBy is defined on a supertype. Please file a ticket for this problem.
I am not sure, but this could be a constraint of the internal database. Did you try also in postgres?
[EDIT] Sorting on owner or changed by have been given problems. Even in Mx 4.8.9 doing an CSV or Excel export on a grid with these columns can get you in trouble. First I would try to see if the latest Mx4 still gives you these problems. If so file a ticket.
Caused by: org.postgresql.util.PSQLException: ERROR: column sub3.system$changedb y does not exist Position: 626