Thats a very odd problem, but u haven't really given much info. Thats a very common sort of error message, but it could mean anything. I've often had error messages pointing me to the cost column, when the error has been elsewhere.
Are u sure you didn't take any short cuts on the second unit.
Try making a very basic unit, just copy one unit and rename it (remember to add descriptions etc). See if that works.
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