To reproduce:

enum Enum1:byte
{
E1,
E2
}
public class C
{
public int _id;
public Enum1 e;
}

var query =
from x in collection.AsQueryable<C>()
where x.e == Enum1.E1
select x;
which results in the following exception:
1
Unsupported where clause: ((Int32)x.e == 0)