Unable to integrate OpenProject server

I am not able to connect my OpenProject server with the OpenProject integration. On the settings page I enter my OpenProject host url, but I get an error of “Please introduce a valid OpenProject host name”, with a popup error of “No OpenProject detected at the URL”. I know the url is valid as I can browse directly to it and log into my OpenProject system. Both servers are https. Anyone know what may be causing this? What is the integration app looking for in the url that I don’t have?