IMac can't find software upgrades/updates

I just bought a used iMac (Retina 5K, late 2015 model, Big Sur 11.0.1 installed). Everything works normally (surfing, etc.), but when I click on 'Software Updates' (via the Apple icon in the top left corner of the

screen or the gears symbol), the computer just keeps searching in vain, finding nothing. At a mininum, the latest version of Big Sur should be shown, but there's just no results. I tried downloading Monterey

directly from the App Store, but all it gets me is the same stuck situation next to the gears symbol ("Searching for Updates …" [translated from German]). What's also weird is that programs such as Pages and Numbers are missing on the hard disk, and aren't shown as potential updates, either. I tried both manual and automatic update options, but both didn't work.