On Debian Stretch and some other Linux distributions there is a small issue with ATI Radeon drivers. Basically, the driver gets loaded too late and Xserver will start behaving badly. There will be dkms modesetting is not supported errors or then something more annoying like startx not working at all. Sometimes it is even worse and you might get randomly broken login to desktop. To fix this issue append radeon to /etc/modules .
echo “radeon” >> /etc/modules
Now, when you restart your computer you should be having a working startx and those random issues where you cannot get pass your login screen should be gone,