I suspect you are working in pcbnew-standalone mode.
As most of board- and schematic settings are stored as project-settings you should always work with the main kicad manager, opening (or creating) a project and open the schematic/board from the kicad manager.
Hey, I have the same issue and couldn’t find a “fix” yet. I suspect it to be a permission issue, it works when I create a project in Ubuntu and copy it to my NixOS. Since I recently switched to NixOS I currently copy & paste an old project delete its content. Not too elegant sadly…
Hmm that’s truly strange. It suggests that NixOS imposes some extra permission system on top of the usual Linux ones. Unfortunately it requires someone willing to sleuth through the NixOS filesystem and documentation. Maybe ask around on NixOS forums?