Hi Steven
Indeed upgrading AMD grahics card driver solved the issue!
Upgraded to AMD Radeon Adrenalin 21.5.2 driver (released 06/21/2021) and the sunshine came back 
(maybe good to know: When starting new driver installation, old driver will be removed in a first step. After restart, system remains with a standart windows driver. Start installation again, and new driver will be installed then. Again restart, and everything is fine).
Thanks for your help and happy zwifting!
Now let the sun shine upon the whole world.
