May I ask if you did tried to troubleshoot if maybe the instance with Zabbix installed is returning 403 from the origin host/server?
It might be a temporary issue. Try refreshing page or try using different web browser.
Sometimes I encounter this, but only when the new domain / Website is being recently added to my Cloudflare account.
Therefore, I wait up for the proper DNS propagation (24 hours), and later on even with my Firewall Rules setup, Bot Fight Mode and other secutiry options like Security Level and similar available on a Cloudflare plan, I did not encounter it ever later again, works fine.
Knows to happen from time to time.
In the meantime, I would wait and suggest to check the article from below:
Some other helpful information other users reported: