Latest LIBC on Ubuntu MATE 18.04.6 LTS

That is not recommended because 20.04 LTS is already end-of-life and therefore out of support. It might be better to install 22.04 LTS

That is a well known nVidia problem which also crops up on 18.04 and 20.04 and is nVidia driver related. See this:

Have you tried NOT using the closed source proprietary nVidia drivers and use the open source free Nouveau drivers instead ? They are probably a much better fit for a 9400GT

(b.t.w. Open source drivers are almost always qualitatively better than their proprietary counterparts).