Hi Matgriff,
Our Developers have been looking into a few reports of this issue, and there is a fix for this planned for an upcoming release of the SEP product.
One thing to note is that in our testing, we could only reproduce this behavior when a SEP for Mac client is configured to use a proxy for LiveUpdate access, but that proxy is not reachable from the client. So the fix our Devs have planned will be to make the error more accurate, and indicate that there was a failure to connect, rather than prompting for credentials.
If you believe you're encountering this popup on clients when they are able to reach their proxy, I'd encourage you to please open a Support case. Thank you.