I’ve never seen this problem before and I’ll need to investigate it further. It’s related to Gnome’s gobject introspection.
I’m pleased to see Fedora has packaged Rapid Photo Downloader 0.9.24 on Fedora 33. However when I launched the Fedora packaged Rapid Photo Downloader, the threading module in Python 3.9 threw exceptions (which is bad). I’ve not tested Rapid Photo Downloader with Python 3.9 myself, so I can’t say if this problem is unique to Rapid Photo Downloader or indicative of a wider problem with the current release of Python 3.9 in Fedora 33.
So as of today, unfortunately Rapid Photo Downloader will not run properly on Fedora 33.