Try clearing out your browser's cache/browsing history. When you have done that, close out the browser and re-start it in a fresh session. If that doesn't do it, then try using a different browser. See if the same thing happens.
Cleared cache. Closed Chrome. Reopened. Still cannot access address book. Tried the same thing in Microsoft Edge. Same thing. :-(
I don't think you can fix this yourself then. So I'm escalating it to the forum admins for follow-up.
We have a known issue with getting a 403 Error Message for Zimbra email accounts (old email interface) on Address Book page. This issue has been escalated to engineers and we are in the process of finding a cause. Thank you for your patience.