Sometimes we face this issue and it automatically resolved after 15 to 20 seconds of time. It can happen for many reasons like maintenance mode or even high traffic and services not accessible for that time. Whatever be the reason you cannot predict so, better you can ask firstname.lastname@example.org for help. You need to provide your account details for them to investigate the exact reason. You can also create a support ticket regarding the same.
"Usually this error code along with HTTP 503 status can be returned when traffic is switched between our data centers. It usually can happen at night time when API traffic is minimal and doesn't last longer than 10-15 seconds. In case of such error API also returns Retry-After HTTP header which indicates when client should retry the original request."
Ref : Benjamin already mentioned here: https://forums.developers.ringcentral.com/questions/351/api-overloadedissues.html