Looking at the numbering (5.8-3), you installed the deb package that comes with Debian. I’m not sure what version Debian used to create this package and you might be running a RawTherapee version that is based on the rather old 5.8 (this one). Have a look at Preferences->About, bottom right corner should tell you a bit more about stable/development and version.
If this, running the old 5.8, is the case: You shouldn’t be using that version, It is old, has bugs and lacks support for a lot of the newer gear. All of those are fixed in the latest, all but 100% stable, development version. It also comes with some new features.
You might want to try the latest AppImage (Direct download links to the latest RawTherapee development version.) and see if that solves your problem.
If it does not, no harm is done (just remove the image). Do post some more info if this is the case, including if possible a RAW and sidecar example.