Same here,
The issue is that Zoom for Salesforce is working in Production, but we are unable to authorize/connect Zoom with any Salesforce Sandbox org. I tested this in both our existing QA Sandbox and a brand-new Sandbox, and both return the same error:
OAUTH_AUTHORIZATION_BLOCKED: Cross-org OAuth flows are not supported for this external client app
Since we are using the standard Zoom managed package, and the same error occurs for multiple users and multiple Sandbox orgs, it does not appear to be user-specific.
I also found a recent post on the Zoom Developer Forum where another user reported the same error when trying to authorize the Zoom managed package in a different Salesforce org