[solved] Login Screen text missing

After updating to Ubuntu 18.04 LTS the login screen shows no user names, just the user icons.
In the follow up screen some letters are missing e.g. "Cancel" is written "Ca l ", see screenshots.
https://www.pic-upload.de/view-35312811/ubuntu18.04.login-screen.png.html

If I toggle the text settings "large texts = on/off" the texts appear. (In this dropdown menu single letters are missing, too.)
So I think it might be a rendering problem.

Any hints?
Thanks in advance...

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I am not sure if this is a bug by ubuntu or by me.

The output of dpkg -l | grep dm:

ii bsdmainutils 11.1.2ubuntu1 amd64 collection of more utilities from FreeBSD
ii cups-ipp-utils 2.2.7-1ubuntu2 amd64 Common UNIX Printing System(tm) - IPP developer/admin utilities
ii dmeventd 2:1.02.145-4.1ubuntu3 amd64 Linux Kernel Device Mapper event daemon
ii dmidecode 3.1-1 amd64 SMBIOS/DMI table decoder
ii dmsetup 2:1.02.145-4.1ubuntu3 amd64 Linux Kernel Device Mapper userspace library
ii dmz-cursor-theme 0.4.5ubuntu1 all Style neutral, scalable cursor theme
ii ebtables 2.0.10.4-3.5ubuntu2 amd64 Ethernet bridge frame table administration
ii gdm3 3.28.0-0ubuntu1 amd64 GNOME Display Manager
ii gir1.2-gdm-1.0 3.28.0-0ubuntu1 amd64 GObject introspection data for the GNOME Display Manager
ii iptables 1.6.1-2ubuntu2 amd64 administration tools for packet filtering and NAT
ii libdmapsharing-3.0-2:amd64 2.9.39-2 amd64 DMAP client and server library - runtime
rc libdmtx0a:amd64 0.7.4-2 amd64 Data Matrix barcodes (runtime library)
ii libgdm1 3.28.0-0ubuntu1 amd64 GNOME Display Manager (shared library)
ii liblightdm-gobject-1-0:amd64 1.26.0-0ubuntu1 amd64 LightDM GObject client library
ii liblouisutdml-bin 2.7.0-1 amd64 Braille UTDML translation utilities
ii liblouisutdml-data 2.7.0-1 all Braille UTDML translation library - data
ii liblouisutdml8:amd64 2.7.0-1 amd64 Braille UTDML translation library - shared libs
ii libnfsidmap2:amd64 0.25-5.1 amd64 NFS idmapping library
ii libpod-readme-perl 1.1.2-2 all Perl module to convert POD to README file
ii libquadmath0:amd64 8-20180414-1ubuntu2 amd64 GCC Quad-Precision Math Library
ii librdmacm1:amd64 17.1-1 amd64 Library for managing RDMA connections
ii libwildmidi-config 0.4.2-1 all software MIDI player configuration
rc libwildmidi1:amd64 0.3.8-2 amd64 software MIDI player library
ii libwildmidi2:amd64 0.4.2-1 amd64 software MIDI player library
ii libxdmcp6:amd64 1:1.1.2-3 amd64 X11 Display Manager Control Protocol library
ii libxdmcp6:i386 1:1.1.2-3 i386 X11 Display Manager Control Protocol library
ii lightdm 1.26.0-0ubuntu1 amd64 Display Manager
ii passwd 1:4.5-1ubuntu1 amd64 change and administer password and group data
ii policykit-1 0.105-20 amd64 framework for managing administrative policies and privileges
ii python3-padme 1.1.1-2 all mostly transparent proxy class for Python 3