RingCentral for Salesforce Adapter Not Loading
Check console and HAR logs from the browser. Verify if the console and HAR logs show that a request for login.salesforce.com/17181/logo180.png gets a response of 404 Not Found. If you get the same response, the problem may be Clickjack protection in the Salesforce settings.
Clickjack protection prevents the framing of content from another domain. Since the RingCentral app is in a Visualforce page from a different domain, if clickjack protection is enabled, the RingCentral app will not display.
Disable Clickjack protection
1. From Setup, enter Session Settings in the Quick Find box, then select Session Settings.
2. In the Clickjack Protection section, deselect Enable clickjack protection for customer Visualforce pages with headers disabled.
3. Save your settings.