Graphical issues under GDM - unreadable/unseeable interface

[Christian]

Since kernel
"Linux U20 5.8.0-50-generic #56~20.04.1-Ubuntu SMP Mon Apr 12 21:46:35 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux"
I am no longer able to use the default Ubunutu Desktop GUI (gdm). Program windows keep flickering when I try to move any window or even only my mouse. Newly opened windows are partially not visible. Black areas keep appearing over elements - or text of any open window. These black areas are also visible on screenshots ... see attachment.

Reproduceability: 100%

Workaround: Use kernel 5.8.0-48-generic

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.8.0-50-generic 5.8.0-50.56~20.04.1
ProcVersionSignature: Ubuntu 5.8.0-50.56~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-50-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Apr 26 07:35:42 2021
InstallationDate: Installed on 2020-04-28 (362 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: linux-signed-hwe-5.8
UpgradeStatus: No upgrade log present (probably fresh install)