The following query results in a "Conversion failed when converting data and/or time from character string" as soon as more than 1 id is selected. If "and id in ('5','6')" is added to the end, the query will provide the ids. If omitted and the same top 2 ids are selected, the conversion fails. This enrollmentdate is stored by the software as an nvarchar - format MMDDYYYY.
declare @yesterday datetime
set @yesterday=cast(DATEAdd(DAY,-1,getdate()) as DATE)
select id from customers
substring(EnrollmentDate,3,2),'1990-01-01')as DATE) > @yesterday
--and id in ('5','6')