Looking at the closed pull requests on the project's Github, the issue appears to be fixed now. Using the "List" option for "View As" appears to be working in my limited, independent testing (tested using API v57.0 and v58.0).
workbench.developerforce.com is hosted on Heroku (the "developerforce" instance from here on), which in turn uses Amazon's AWS (hence "amazonaws" appearing in the error message).
According to the pull requests, the issue was related to Redis v6 (developerforce appears to use a Heroku add-on, "Heroku Data for Redis", as a session store for operation across multiple compute resources, or "Dynos" in Heroku parlance) and making a TLS connection.
Heroku started requiring TLS for connections to its Redis add-on for all but the "mini" instance (where TLS is supposedly only suggested rather than required) when using Heroku Data for Redis v6. URLs and port numbers may have changed, and this add-on uses a self-signed cert (which required some additional changes in setting up the TLS connection from PHP, namely disabling peer verification).
There was also apparently some work that needed to be done because the session save handler (in the Redis driver that PHP provides?) isn't able to handle TLS connections yet.
Shoutout to Ryan Brainard for putting in the work on this.