I got the same error message when attempting to set the path to the Affinity Photos.exe, so it seems like it's a Windows permission issue. I was able to discover a workaround. I launched Affinity Photo and then opened the Task Manager. Open the Affinity Photo 2 entry and then right-click the Affinity Photo entry, Now click the Open file location entry from the contextual menu. This will open a new window with access to the Photos.exe app. Right-click and drag the Photos.exe icon to the Desktop. Use the Create shortcuts here option. Now rename the shortcut to just Photos. Now you will be able to set the path to this Desktop shortcut on the Launching preference page. Affinity Photos does not need to be running in the background. The app will launch when sending the selected images to it from PM.
--Bob