(- delete various gamefiles, e.g. delete CryRenderD3D9.dll, CryRenderD3D11.dll, mwoclient.exe, or frontend.dll, in fact I deleted them all
- disable antivirus/firewall)
and nothing helped to get the patch going.
What I did is to unistall the game via "programs and functions" of the windows control panel - the game only had about 4 MBs in size listed there.
After the "uninstall" I only had to download the latest patch (about 80 MB) and all worked fine

No "complete" reinstall needed.
Maybe this works for other people as well.