Comment # 3 on bug 1223900 from Giacomo Comes

I figured out what exactly the problem is.

redshift provides a apparmor profile: /etc/apparmor.d/usr.bin.redshift
such profile include: <abstractions/X>

Which in turn grant access to the xauth file:
  owner @{HOME}/.Xauthority r,
  owner @{HOME}/.local/share/sddm/.Xauthority r,
  owner @{run}/gdm{,3}/*/database r,
  owner @{run}/lightdm/authority/[0-9]* r,
  owner @{run}/lightdm/*/xauthority r,
  owner @{run}/user/*/gdm/Xauthority r,
  owner @{run}/user/*/X11/Xauthority r,
  owner @{run}/user/*/xauth_* r,

sddm up to 0.19 was putting the xauth file in:
  @{run}/user/*/xauth_*
Since 0.20 the xauth file is located in: /tmp/xauth_[A-Za-z]*
which is not included in any apparmor's abstracion file.

Possible solutions are (in my preferred order):
1) revert the change in sddm and put the xauth file in the previous location
2) add in the apparmor's profile abstractions/X an entry for sddm's new xauth
file location
3) add in redshift's profile such entry.

I can prepare a SR for redshift to fix the issue but that's only a workaround.
Some other programs that are using abstractions/X profile to allow access to
the user's xauth file will have the same problem.
Solution 1 or 2 is the appropriate fix.