MongoDB and Robomongo: Can't connect (authentication)

Robomongo 0.8.x doesn't have support for the new SCRAM-SHA-1 authentication which was the default authentication method added in MongoDB 3.0.

Robomongo 0.9.0 RC4 (Feb 2016) added support for SCRAM-SHA-1, but if you are using a modern version of MongoDB server you should upgrade to the latest version of Robomongo (now Robo3T) currently available.

There is also an officially maintained free GUI, MongoDB Compass, which is more actively updated than Robo3T. MongoDB Compass 1.22+ (Sept 2020) includes an embedded MongoDB shell.

I was experiencing exactly the same issue on Mac. I removed the Robomongo client and download the latest version from their website. It just works :)