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. The upgrade should correct the issue.