This error indicates the super global PHP variable value of $_SERVER["HTTP_USER_AGENT"] is too long for the "p_requests" table column "agent". The visitor's HTTP_USER_AGENT value is greater than 255 characters, which is rare and doesn't happen often.
As of PHP Live! v. 4.1.84, the table "p_requests", column "agent" has a default length of 255 characters. Check your PHP Live! version and be sure it is v.4.1.84+. If the version number is lower, download the latest build from the client area and upgrade your PHP Live! system. After upgrading, if the error still exists, increase the column "agent" from the table "p_requests" to 280 length using a MySQL front-end tool (such as PHPMyAdmin).