Flatpak packages can't access the rest of the filesystem by default, so that might be getting in the way of things.
You can create a rule allowing specific Flatpak packages access to certain files, however, using something called Flatseal (available in Software Store):
Scroll down to Filesystem and either enable All user files, or specify the file to the location that you need. From what I've seen plugins should be stored in /home/zenzen/.local/share/rhythmbox so that's what I'd suggest as that's all you need. But you can also enable it for your entire home folder if this doesn't work out.