If user cancels "Assign Item" command performing on the IAM file any further attempts to repeat the command on that IAM during that same day would fail with error 155.
Official recommendation for the issue (http://crackingthevault.typepad.com/crackingthevault/2012/12/error-155-illegal-null-parameter-during...) seems inadequate (keep reading please).
The only known reasonable workaround is to repeat command on next day (it always helps).
The root of issue I believe is some "draft" Item that is not present in Item master during the first day and that is auto-deleted at night.
If you like you can even see that draft Item using SQLquery:
use [AAAA]
go
declare @MyItemID numeric(38)
declare @MyItemNumber nvarchar(50)
SET @MyItemNumber='BBBB'
SET @MyItemID = (SELECT [itemMasterID] FROM [vw_ItemIteration] where ItemNumber = @MyItemNumber group by itemMasterID)
SELECT [ItemNumber] FROM [ItemNumber] where ItemMasterID = @MyItemID
Where
AAAA - name of the VaultDataBase
BBBB - expected "Number" of Item
I wonder if there is any way to delete the draft Item and not lose the whole working day?