Why can I not establish a connection with RingCentral server?

It's hard to say what could be the cause of this problem.

Had this problem happened before when you test it on sandbox? Is the app now running on the same system (browser, OS, internet connection, firewall etc.)

Can you give more details of what type of authentication (password flow or 3-legged authentication), APIs you call from the app etc.

+ Phong

We already use password flow:

await rc.Authorize(Username,Extension, Password);