I get the following error when I log in with an useraccount which has some products in the cart:

warning: pg_query() [function.pg-query]: Query failed: ERROR: operator does not exist: character varying = integer LINE 1: DELETE FROM uc_cart_products WHERE cart_id = 1 ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. in /var/www/drupal/includes/database.pgsql.inc on line 125.

I found the error on line 590 in uc_cart.module

db_query("DELETE FROM {uc_cart_products} WHERE cart_id = %d", $uid);

it has to be

db_query("DELETE FROM {uc_cart_products} WHERE cart_id = '%d'", $uid);

because cart_id is not a number.