For Heroku apps created on June 14 2023 or later, Heroku now adds some random characters to the app's URL.

So instead of my-experiment.herokuapp.com, the URL looks like my-experiment-37bc129aef.herokuapp.com.

More info:

https://help.heroku.com/7KRHKEPA/why-are-there-random-characters-in-my-subdomain
https://devcenter.heroku.com/articles/app-names-and-subdomains#subdomains

Apparently this can be customized, but I haven't tried it.

I have fixed this on oTree Hub's end to recognize the new URL format. To ensure oTree Hub has the up-to-date URL for your apps, load this page: https://www.otreehub.com/my_projects/