External monitor not detected after login, but working after re-login

I am using an external DELL monitor connected by HDMI to my HP zbook notebook with RADEON graphic card. After the first login, external monitor stays black with the “_” sign in the top left corner. External monitor is even not detected by xrandr. When I log out and login again, everything works fine. After reboot and first login, again black screen.
I tried to switch off all autostart applications in i3 .config file except for xrandr, but it doesn’t help. It is not a big issue, but makes it less user friendly. Many thanks for suggestions.