Thanks for the help. I tried all that and it didnt help (but those were good suggestions!).
Then I proceeded to uninstall, reinstall, uninstall, reinstall, uninstall, reinstall, installing on another harddisk drive etc etc, fiddling with graphics settings, restarting my computer uninstall, install again again. Suddenly it worked. Sadly I have no idea what the hell I exactly did to suddenly make it work again.
All I can say is: the game must be hiding some textures, or shaders or whatever, somewhere else than the actual games installation folder. And these files, whereever they may be, are NOT deleted upon uninstallation (obviously, since simply uninstalling and making sure that actual FEC folder is deleted and then installing game again did not get rid of the issue.). But maybe somehow along the way when you reinstall, fiddle with graphics settings, etc these files may get updated, deleted, overwritten or whatever is really actually happening.
I would love if the developers could inform us WHERE they store these secret texture files so that I may easily fix it by deleting them should it happen again in the future. But if that doesnt happen I may start monitoring the Frontend.exe process with Windows task managers ressource monitor in the disk section to see if I can spot access to files on the harddrive that may have something to do with textures/shaders etc.