The problem is that your application is running under an elevated privilege level, possibly due to a miss-configuration during installation. I.e. your app is running as administrator (high), and Explorer is running at a medium level. This stops drag and drops from explorer to your app.
See here for more information:
I was going to suggest the following link as a remedy:
It would seem that this isnt possible as I think Xojo uses OLE, which cant be circumvented.
However, fixing the installer might be the way to go, you need to lower the privilege level of you app.
You can see the same problem if you run Notepad as administrator (or Xojo for that matter), you cant drop a text file onto it, however you can do so if you run notepad normally, your program is having the same issue.
As a quick thing to check is the properties on your exe (right click, properties) and go to the Compatibility tab, look in the Settings section and ensure that “Run this program as an administrator” isn’t checked at the bottom of the pane.
But yes, check your Inno installer config, good luck
PS. It could be where the app is placed, certain folders run as administrator, could be worth checking
PPS. Turning off UAC will also get around the problem, but that might not be wanted, just an option