Asking this under general discussion before I file a bugreport, because the error seems like it might be on the TVDB/Sonarr side of things, rather than the software install (and also because it keeps happening, even on a separate RPi 5 with fresh Dietpi install, and I can’t find any obvious issues with the install itself).
[edit] I guess I’m not alone; Failed to load series from API · Issue #8249 · Sonarr/Sonarr · GitHub though this error isn’t exactly what I’m seeing.