Hi Justin,
Seems like you are using function? Is it a user defined function or default function of SQL Server?
Removing BusinessObjects from picture, if you execute the same query directly against the database , does it give the same error or does it run fine?
If you get the same error against database then you need to check the below defintion:
dbo.fnc_users(BAM_ENTRY.LAST_VALIDATED_BY_GUID,'name')
If it is a user defined function, you can consult your DBA and check on its functioning. It seems like there is some value 'a00070' which is not getting converted by the function fnc_users.
Regards,
Yuvraj