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I found this old question to a problem I am experiencing. My application uses "Password-based auth flow" and "Issue refresh tokens" is set to "Yes." Every so often I receive "Refresh token is missing." Sometimes my application recovers from this, but other times I have to restart the application to get it to work again. I never know when I will receive it. I have had the application run for a few days without seeing the problem. I have also seen it pretty quickly after the application starts.

My Client ID is 2Jq2niD7TFev7x8voo8Cdg

I don't think that this is the same issue as your app is password flow. The issue in this original question is about the RC embeddable which requires 3-legged authorization.

In your case, I am sure that the issue happened when you are testing different apps (your codes) which use the same RC app client id and login with the same user.

Hi @Andrew Prokop It is recommended to use 3-legged-OAuth flow (Javascript) with refresh flow for the Embeddable widget. With other flows, you need to pass app client secret to the widget for refreshing token.
Can you help to create developer support ticket here https://developers.ringcentral.com/support/create-case with your Embeddable version, client id, logs and HAR file.They can help us track the issue.

To collect HAR file, you need to open Chrome developer tool first, then reload the page to reproduce the issue. Then in network panel, right click to save all as HAR with content.

@Embbnux Ji I've written a server application that doesn't need a user interface. From what I can tell from the RC application creation process, that's where I should be using a Password-based auth flow.

If you wrote a server application that doesn't need a user interface, why and how do you use it with the RC embeddable?

I am sorry if I caused confusion. I tacked my question onto this one only because I was receiving a "Refresh Token Missing" error, too. That was the similarity. I didn't want to create a new question if it was basically the same problem. I see now that it is different enough.

That was why I wrote it is not the same issue and I also gave my suspected reason. This happens to me too, many time. E.g. I have an app running on a remote server using password flow and it would refresh the access token using the refresh token. But every time I test some other code locally using the same app client id/secret, my remote app would stop working because the refresh token become invalid due to the local authentication several times which causes one of the existing tokens to be revoked.
In your case, I am sure that the issue happened when you are testing different apps (your codes) which use the same RC app client id and login with the same user.

That could have been my case. I had an old application running in the background that used the same ID and Secret. I've killed it and will wait and see if the problem shows up again. Thanks for your assistance!