You apparently did not run the vssadmin command in elevated mode. You have to right click on cmd in accessories and run as admin. Else it will not work.Thanks for reply, I posted my screen shot. Also tried vssadmin list shadowstorage, but I'm afraid that It only opens a window for short time, so I couldn't post any output here.
Yes, I have a 60 gb SSD. I have tried your advice and changed all browser download locations to another drive, but everything is still the same. Beside that, default folder for browser is "Download" folder in Explorer and I delete all files there manually each week, so that shouldn't be problem.
Maybe my SSD is performing something like "hidden" cache ?