set pruduct weight then checkout page error

[haoyida]

I set the pruduct weight to larger than 0, then the checkout page error appears, as the above image.
I use zencart v1.39e and big_royalmail_v2.3.8 for shipping.
how is the weight related to soapclient?
why error appears, please help, thanks

[haoyida]

the number on gray background is the weight I set. if I change the number of weight, the number in error message will change accordingly.

[haoyida]

the big_royalmail uses kg as unit of weight, zencart uses g as unit of weight, I have tried to change the unit in the under pages
/includes/languages/english.php
/includes/languages/english/modules/shipping/zones.php
from g to kg.
but the checkout page error remains.

[haoyida]

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