you should be able to fix it without a trace at all; the error is in the report itself.
somewhere in that' reports definition,you've got something like
...WHERE ClientID = (SELECT ID From AnotherTable)
for that one client, that table has more than one row, and causes the error.
grab the definition of the report, and find it.
simply changing it to an IN would fix it immediately.
...WHERE ClientID IN (SELECT ID From AnotherTable)
Lowell