Getting an error when trying to connect to a Sandbox with Workbench.
It was a known issue: https://github.com/forceworkbench/forceworkbench/issues/824
Looks to be resolved now.
IP Restrictions appear to be enforced by default on the new Workbench App connected app. The org I'm connecting to does not have IP Restrictions setup... changing the IP Relaxation field to "Relax IP restrictions" allows us to login.