I had a look at the config.json that seems to get added to the Build, and saw that dataUrl/frameworkUrl/codeUrl were not set. I created my own config.json, referencing the correct uris, and reuploaded it.
Now I get a new error message
"Uncaught ReferenceError: UnityLoader is not defined"
After having a look at Changes to the WebGL loader and templates introduced in Unity 2020.1 it looks to me as if UnityPlay is searching for the older way of loading the game. But the new way was introduced with 2020.1, so I guess UnityPlay should be compatible with it?
I might miss something obvious, so sorry if that’s the case.