Connection security with Marketo email links

Hi all, 

While testing the links of a Marketo email, one of my colleague got this message on this browser  (Macbook + OS11.4 + Chrome or Firefox) "Your connection is not private" - similar to this https://nation.marketo.com/t5/knowledgebase/tracked-email-links-return-your-connection-is-not-private/ta-p/250847

All the links work fine for the rest of the team so I believe the branded tracking links starting are secured. We are not sure if we should send the email in case other customers see the security message.

Any idea what could have possibly happen?