I cannot seem to find out how to track down this bug: I can see it has something to do with Shipping Rates, but there’s very little information being posted into the error log.
2013-01-08T15:45:36+01:00 DEBUG (7): requestShippingRates
There are many of these errors being posted.
If you have shell access, try using the following command to search for instances of requestShippingRates. Run it from your /var/www/app directory. the * means look at everyting, the -R means recurse directories, and the -i means ignore upper / lowercase.
You did not mention whether or not these errors are showing up in your php log, but you could also grep the following:
That will show anything written to the php error log.