@simos the problem only occurs in the webui. i have installed the virt-viewer on my mac, switched with incus remote switch ... to incus server and could start viewer with incus console w11 --type=vga successfully. From now on it is also possible to use the graphical console in the webui.
for the installation of virt-viewer on my mac m1 i used:
brew tap krishjainx/homebrew-virt-manager
brew install virt-manager