Ccleaner did not run at startup.
When I tried to run GoG Galaxy as non-admin, nothing didn't even happen.
But when I did run it as admin, Task Manager briefly showed up GoGGalaxyClient together with a Ccleaner app/service. After a couple of seconds both disappeared.
Task Manager showed no reference to Ccleaner being active whatsoever.
When I uninstalled Ccleaner and re-installed GoG (as admin) everything worked (and still does so) perfectly.
I don't even have to run GoG as admin any more either.
So saying Ccleaner has nothing to do with it is an untruth.