Fresh reinstallation of windows 10 pro, previously Dropbox worked just fine. I start the setup, it downloads the app and just hangs on loading indefinitely. Then I tried to restart PC, it won't login to my acc - just black screen but I see the mouse cursor and can open menu with ctrl-alt-del to sign out. I could go into another account on this PC without problem. Registry clean with CCleaner and suddenly I can go back into my acc. I gave up after few tries.
After a couple of days I try again, this time the setup went perfectly, my account works. Problem is that now I can't login into other account with the same simptoms. Uninstalling dropbox, ccleaner - problem solved.
Only thing that is different than previous installation is that I installed Python.