Developer API SSO Default Password

As per the links here, if you are facing any issues with password, you can contact RingCentral Support for reset of password

https://support.ringcentral.com/article/6748.html

https://support.ringcentral.com/article/10063.html

I am asking with regard to the developer sandbox; whether it's my SSO password or something else.

Have you setup your sandbox to support SSO? If not, you have to do the setup first. Login your sandbox service web with the admin user, then run the setup. I am not quite sure about your last question "I can't seem to reset my sandbox password either.". How this is related to the first question?