The wifi that the bidders are logging into has some safety filters on which is causing the errors that bidders are seeing when they log in. Bidders can safely ignore the error message and continue. They can also choose to shut the wifi on their device off and use cellular data.
Sample Error Message
When you send your landing page URL through a mail service that has click tracking enabled, it changes the URL in the email that is received to something that we may not be able to convert to secure.