we have integrated the RingCentral Embeddable widget to our web app. We are currently testing it with our sandbox account and we noticed, that one of our users cannot sign in using the widget. When he clicks "Sign In" button, everything seems correct - new browser tab is opened where he enters credentials. Afterwards it notices him about app scopes, which he authorizes. Then the tab is closed and the original tab where he clicked the "Sign In" button shows "Login failed due to internal errors. Please try again later." error message instead of having the refreshed widget with logged in user.
I've noticed only one difference between users for who it worked and for who not - when user who was able to sign in clicked the "Sign In" button, it opened a new window / popup, while for the other user it opened a new tab. Both users use same Chrome browser. The app should be configured correctly in the RingCentral account as it worked nicely for the other user - he was able to sign in, make & receive calls, etc.
Could you please advice what could be causing the issue? This is how the widget looks like after the tab is closed (after signing in).