In xfce I use redshift to adjust the monitor night color.

If I use lightdm as displaymanager, redshift works normally.
Typing the command:
  redshift -o
gives the output:
  Trying location provider `geoclue2'...
  Using provider `geoclue2'.
  Using method `randr'.
  Waiting for current location to become available...
  etc...
But if I use sddm as displaymanager, redshift doesn't work.
Typing the command:
  redshift -o
gives the output:
  Trying location provider `geoclue2'...
  Using provider `geoclue2'.
  Authorization required, but no authorization protocol specified
  Authorization required, but no authorization protocol specified
  `RANDR Query Version' returned error -1
  Initialization of randr failed.
  Trying next method...
  Authorization required, but no authorization protocol specified
  Authorization required, but no authorization protocol specified
  X request failed: XOpenDisplay
  Initialization of vidmode failed.
  Trying next method...
  No more methods to try.

The output of:
  strace redshift -o
contains:
  access("/tmp/xauth_WYBTin", R_OK)       = 0
  openat(AT_FDCWD, "/tmp/xauth_WYBTin", O_RDONLY) = -1 EACCES (Permission
denied)

The failure of redshift is due to the fact that somehow it cannot access
/tmp/xauth_WYBTin (the value of $XAUTHORITY)
However such file is readable by the user running redshift and other xprograms
work fine.

Leap 15.5 (sddm 0.19) works fine as did the previous versions.
The issue started with sddm 0.20
(https://build.opensuse.org/package/show/openSUSE:Factory/sddm?rev=64)
and is currently present in Leap 15.6 and tumbleweed.

I could not figure out what exactly in sddm 0.20 is causing the problem.