GitHub includes an IP restriction feature that allows Organization Owners to configure a list of allowed IP addresses (also known as a whitelist) that can access the repositories within their Organization.
When this feature is enabled, it disrupts your ability to use Zenhub within that Organization because ZenHub's backend servers are not on the whitelist and cannot access the GitHub API on your behalf. To resolve this issue, you can add Zenhub's NAT IPs to your whitelist. For a current list of Zenhub's NAT IPs, please contact Zenhub Support.