Scenario

You're using a firewall that is using SSL interception. All required ports were opened as per http://support.kaspersky.com/13326 . On the corporate network, activation fails with "error 6". Off the network, activation is successful.

Solution

The reason this might not work is that your SSL-intercepting firewall (3rd party) might be attempting to check the validity of the SSL certificate that protects Kaspersky's activation server (activation-v2.kaspersky.com, TCP port 443). In particular, the firewall might be trying to check whether the certificate is revoked. Checking a certificate's status can be done through an older protocol called CRL or a newer protocol called OCSP.

The fix is to configure firewall to not perform SSL interception relating to TCP port 443 of activate.activation-v2.kaspersky.com.