Quite sure it isn't a problem with arrays as I also get the error
(again intermittently) with this data  type:

Public Type TypeTRA
TypeID As Byte
Ref As String * 8
Date As Date
StockCode As String * 20
Qty As Integer
LocFrom As String * 9
LocTo As String * 9
End Type

The value returned by Len is not a problem, it is the same whether
the code  works or not.

I'm inclined to believe that there is some internal VBA bug  that
corrupts the data type structure in some way.