So, I removed/deinstalled CD completely - that means with an external uninstaller in addition, deleted manuel the rest, andprefetch files. Now I can install the extensions again.
Virtual Mode doesn’t use the normal profile of the browser stored in %localappdata%\Comodo\Dragon\User Data. It’s actually a “sandbox” folder stored in the same partition the OS is installed. You can look for the folder named VTRoot there (it’s hidden by default so you have to turn on “Show hidden files, folders and drivers” in File Explorer Options). In that folder there should be one or more HarddiskVolume* folder(s) containing a mirror image of the folders that store application data on the normal Windows folders (e.g. AppData and LocalAppData). You can find the Dragon or other browsers profile there and delete them.
There may be extensions do require third party software or needs to write files in temp that may not work as intended on Virtual Mode because of security restrictions though.