Hello, I am working on 3CX integration with Zendesk. We are using client-side integration, since it is more convenient for our agents to have tickets automatically updated after phone call. We have 2 Zendesk instances - Sandbox and Production. I have prepared integration according to tutorial on both instances: Code: https://www.3cx.com/user-manual/zendesk-crm-integration/ - document no longer exists Problem is following: When I am trying to log in in Production environment, I receive following error: When I am logging in Sandbox environment using add-in, it works: Zendesk support told me that problem is related to 3CX add-in, not instance. OAuth authorization is configured correctly, account credentials are correct. How to fix this login problem? Please help me.