I would guess it is a library version mismatch. If you have made your projects in kicad 4 then they where made with the kicad 4 footprint lib which expects the kicad 4 3d model lib.
So download the version 4 3d model libs via the library download page. Put them somewhere convinient and point KISYS3DMOD to that place while working on old projects.
For new projects it is generally suggested to use the new lib as it comes with a lot of improvements. (There even exists a backport to kicad 4 for people who can not update to kicad 5)
If you had kicad 4 installed previously then i would bet you still have parts of its configuration surviving. Most importantly the fp-lib-table. This file controls which footprint libs you use. If this is the case then you propably still run the on demand online footprint libs of version 4. But the kicad installer replaced the local symbol and 3d libs with the ones from version 5. This results in another mismatch in libraries.
It is easy to discover if this is the case. open the library manager in pcb_new and check if you have any lib with the prefix Housings (example Housings_DIP) if this is the case you run the version 4 footprint libs.
You now have two options. Install the full version 4 library (replace the symbol and 3d models that where installed with version 5 of kicad) This would allow you to work with old projects easily but locks you in the old lib that is no longer developed.
Or you update the fp-lib-table to work with the version 5 footprint libs. The safest way is to open the library manager again, delete all old libs (the ones starting with KIGITHUB) and then use the browse button to add the new libs (they are found under
C:\Program Files\KiCad\share\kicad\modules for windows users. You can use shift left click and crtl left click to select multiple libs at once. Only select .pretty directories.)
The second option is to delete your fp-lib-table from your config directory (in
C:\Users\ …\AppData\Roaming\kicad). When you then start kicad (or pcb_new from the project manager) it should ask you if it should create the default library table. After this step you will need to manually add your personal libs again.