Thank you for your reply...
But it gives:
Error 182 - Radeon ™ Software Install Detected AMD Graphics Hardware in Your System Configuration That Is Not Supported With This Software Installation
Indeed, I believe that it is an on-board card (M series ...) and the AMD site indicates that you have to go get the driver ... but the latter stops at Win8 (https://www.amd.com/en/support/embedded/amd-embedded-radeon/amd-radeon-hd-series/amd-radeon-hd-6970m...) ...
I tried to install the "Auto-Detect and Install Updates for Radeon ™ Graphics and Ryzen ™ Chipset Drivers for Windows" but it gives me an error 173 indicating that no AMD graphics hardware is installed on my system ... (why?).
Thanks again if you have any other ideas ...
The drivers for apple computers are even under Bootcamp running Windows are supplied by Apple. Any issues with them is reported to Apple. Contact Apple support with issues with your iMac.
I your case you have graphics that even if it were PC only are still no longer supported. Every time new Windows 10 versions come out the chance of your system no longer working increases as it will no longer get driver updates. If Apple has no advice then go to an apple bootcamp forum. Fellow users might have some ideas we would not know here on how to get older macs working in bootcamp.
The generic Windows driver is supplied by the Windows repository. However many of those don't work in bootcamp. That setup is unique to Apple.
Moving forward I would not expect updates on bootcamp either. Apple is currently abandoning x86 in favor of their own ARM processors and no longer offers bootcamp support on the new devices. It will all likely be unsupported within 3 years. That is just my guess based on their past support strategies. Apple never seems to give a date they just abruptly end support.
Thanks for your help.
In the meantime, I found a workaround that seems to hold, I renamed the igdkmd64.sys file outright to igdkmd64BackUp.sys! As I use my computer only for Internet and work software, I don't notice any differences in the display of the screen ... I will continue to look for a more satisfactory solution.