I think thta KDE's file should have 600 too like the origin - even if /tmp/kde-<user> is set to 700, or ? :
$ ls -l /tmp/kde-tfoerste/xauth-1000-_0 ~/.Xauthority
-rw------- 1 tfoerste users 207 Jan 4 22:13 /home/tfoerste/.Xauthority
-rw-r--r-- 1 tfoerste users 207 Jan 4 22:13 /tmp/kde-tfoerste/xauth-1000-_0
$ md5sum /tmp/kde-tfoerste/xauth-1000-_0 ~/.Xauthority
i have no clue what is creating /tmp/kde-tfoerste/xauth-1000-_0, but it certainly isn't kdm itself.
(In reply to Oswald Buddenhagen from comment #1)
> i have no clue what is creating /tmp/kde-tfoerste/xauth-1000-_0, but it
> certainly isn't kdm itself.
See also bug 358593.
KDM is unmaintained and not used in KDE Plasma 5.
SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/