The scenario: I had my RingCentral soft phone installed, and it was configured to use my Development (Sandbox) account of RingCentral (as documented here: https://developers.ringcentral.com/library/tutorials/test-account.html). Earlier in the day, I installed the most recent update (as recommended) to the software. When I tried to login later, I was unable to do so (no matter what account I tried using, or if I modified/removed the config.ini which allowed me to use my RC-Sandbox account). I kept receiving an error message "Invalid Credentials". I tried using the SSO (Single Sign-On), but that failed too.
The fix: I simply uninstalled the RingCentral for Mac software from my MBP (Move the Application to the trash), and downloaded the latest version from the web ( http://downloads.ringcentral.com/sp/RingCentralForMac) and re-installed. Then all was well with the world and I was back to developing.
Hope this helps if you run into the same issue.