same problem. thanks nicolas for directing me to this thread.
problem started in the last 1-2 days only, with the following in error log:
[Mon Nov 11 13:04:55 2013] [error] [client 72.28.152.198] PHP Notice: Trying to get property of non-object in /var/www/vhosts/default/htdocs/plugins/hikashopshipping/usps/usps.php on line 224, referer: emotorwerks.com/products/online-store
[Mon Nov 11 13:04:55 2013] [error] [client 72.28.152.198] PHP Notice: Trying to get property of non-object in /var/www/vhosts/default/htdocs/plugins/hikashopshipping/usps/usps.php on line 231, referer: emotorwerks.com/products/online-store
[Mon Nov 11 13:04:55 2013] [error] [client 72.28.152.198] PHP Notice: Trying to get property of non-object in /var/www/vhosts/default/htdocs/plugins/hikashopshipping/usps/usps.php on line 231, referer: emotorwerks.com/products/online-store
[Mon Nov 11 13:04:55 2013] [error] [client 72.28.152.198] PHP Fatal error: Call to a member function xpath() on a non-object in /var/www/vhosts/default/htdocs/plugins/hikashopshipping/usps/usps.php on line 239, referer: emotorwerks.com/products/online-store
I doubt that the problem is caused by anything on hikashop's users' side (e.g., hosting company blocking outgoing connections etc) given that we all independently have the same problem appearing out of the blue around the same time.
For now, we have disabled USPS plugin which means we cannot accept international shipments.