I have a (stable for years) Kubuntu 16.04 installation VERSION="16.04.4 LTS (Xenial Xerus)"
Recently, I set up for the first time a second user account, and used "new session" to switch to the other user. On the second user's login screen, below the password field, the button said E t rren (or similar) where it should read Entsperren (German for unlock). So some glyphs are missing.
Also, the clock misses some digits:
Please note that the date should read 08.05.18, so the first digit zero is rendered, the second is not. I can adjust the toolbar height thusly resizing the clock pixel by pixel, and random digits keep disappearing and reappearing, but only when the have the approximate size showing in the screenshot. E.g. reducing the height lets the date appear normal but the time digits disappear.
Additional note: resizing the toolbar and thus the font size makes other random characters vanish, but restoring the size lets always the same
characters disappear. E.g. in the example above shrinking the date font 1 pixel lets it display 0 . .18, adding 1px again and it displays 0 . 5.18again.
I have tried to reproduce this using LibreOffice trying to select a font that might by used by the clock, but could not find any rendering problem.
Also, only a few applications seem to show the problem, among them the welcome screen and the clock widget.
I have never seen this problem before switching to another user's session for the first time.
Rebooting now keeps showing the problem, even when I never activate another user session.
Not sure if it is linked to Random Missing Characters on Ubuntu + Unity + External Display or How to fix display issue where random characters are missing? however at least the font and size appears alike.
Update: I installed Kubuntu 18.04 and (while not showing up at the beginning) it also sports the missing characters problem:
So it seems reasonable that maybe the graphics card driver may be the problem (it is the same version 390 in both Kubuntu versions):
$ dpkg -l | grep nvidia
ii libnvidia-cfg1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA binary OpenGL/GLX configuration library
ii libnvidia-common-390 390.48-0ubuntu3 all Shared files used by the NVIDIA libraries
ii libnvidia-compute-390:amd64 390.48-0ubuntu3 amd64 NVIDIA libcompute package
ii libnvidia-compute-390:i386 390.48-0ubuntu3 i386 NVIDIA libcompute package
ii libnvidia-decode-390:amd64 390.48-0ubuntu3 amd64 NVIDIA Video Decoding runtime libraries
ii libnvidia-decode-390:i386 390.48-0ubuntu3 i386 NVIDIA Video Decoding runtime libraries
ii libnvidia-encode-390:amd64 390.48-0ubuntu3 amd64 NVENC Video Encoding runtime library
ii libnvidia-encode-390:i386 390.48-0ubuntu3 i386 NVENC Video Encoding runtime library
ii libnvidia-fbc1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL-based Framebuffer Capture runtime library
ii libnvidia-fbc1-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL-based Framebuffer Capture runtime library
ii libnvidia-gl-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii libnvidia-gl-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii libnvidia-ifr1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL-based Inband Frame Readback runtime library
ii libnvidia-ifr1-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL-based Inband Frame Readback runtime library
ii nvidia-compute-utils-390 390.48-0ubuntu3 amd64 NVIDIA compute utilities
ii nvidia-dkms-390 390.48-0ubuntu3 amd64 NVIDIA DKMS package
ii nvidia-driver-390 390.48-0ubuntu3 amd64 NVIDIA driver metapackage
ii nvidia-kernel-common-390 390.48-0ubuntu3 amd64 Shared files used with the kernel module
ii nvidia-kernel-source-390 390.48-0ubuntu3 amd64 NVIDIA kernel source package
ii nvidia-prime 0.8.8 all Tools to enable NVIDIA's Prime
ii nvidia-settings 390.42-0ubuntu1 amd64 Tool for configuring the NVIDIA graphics driver
ii nvidia-utils-390 390.48-0ubuntu3 amd64 NVIDIA driver support binaries
ii xserver-xorg-video-nvidia-390 390.48-0ubuntu3 amd64 NVIDIA binary Xorg driver
