Interesting! I assume you had the device plugged before Rapid Photo Downloader startup, and something is causing a continuous loop of error-retry-error.
I tested it on KDE Neon, which is also based on Ubuntu 20.04, and it worked fine for me.
Please follow these steps to open a bug report, but put it on GitHub and not Launchpad: Rapid Photo Downloader: Documentation. This is the GitHub link: Issues · damonlynch/rapid-photo-downloader · GitHub