Unable to download app

I'm a developer of a Mac app and when I try to download it on a test system, I get this message "Unable to download app"
with the suggestion to try again later. I did that and still get this message. The test system is a MacBook Pro running MacOS 11.3.1.

What else can I try to get some kind of diagnostic of the problem?

An earlier version of the App is running on the system.
Unable to download app
 
 
Q