I am having the same issue. Reading the online help, it LOOKS like we should be able to use the checkbox of 'identity inserts', but as soon as I select a table, this option disappears, even if it was checked. I have checked that the column is the PK, not null, auto-increment. This is in 6.3, this process used to work for me (same column and data) in 6.0.
"Null value cannot be inserted to not null column id"