I don’t have an answer to your current problem, so my apologies for that. But I do have a suggestion if you’re willing to consider an alternative. I use Rhythmbox for music as well, but having tried their podcast option, I honestly always found it underwhelming, slow to load/update, and a bit cumbersome to get working half the time. If you’re using a GTK music player, I’m assuming you’re also a fan of other GTK apps perhaps and for podcasts there’s nothing better than Gnome Podcasts. It’s actively maintained, integrates well with the Gnome environment, and it’s available in the main Arch repos or a flatpak if you like.
I use it daily and never have any problems with it. It’s not fully featured as podcast apps on your iPhone/Android might be, but it gets the job done and there are plans to include more features in future updates. As it stands now though, it does fit my needs, hopefully it’s an alternative that you might like as well. Below is what it looks like.
sudo pacman -S gnome-podcasts
Gnome Podcasts can import any OPML file backup if you have one, if not, you may have to manually enter in your podcasts RSS feeds. There is currently no search feature in the app, so you do have to know what exactly you’re adding. The best site I’ve found to find your favorite podcasts RSS feeds that Gnome Podcasts needs to read is here: