(Reposted link to thread to make this a title)
EDIT: Testing again, this time completely removing all trace of Atmel Studio, directories created, etc. Registry clean and temp files removed (I used "ccleaner").
After a couple of hours, total fresh Studio install. (With exception of the file driver-winusb-installer-7.0.790.msi - problem with amazonaws.com' server I guess...)
Same result as in the above link. Maybe I should've removed Microsoft *.2015? OK, I'll try, but it's a complete pain in the trouser department, as the (un)installer seems to have been written in Java/python/basic, or some other fiendishly slow crap.