Comment # 10 on bug 1104642 from Michael from Offenbach Germany

(In reply to Fabian Vogt from comment #9)
> Please add it back and run the commands again - they assume the line was
> active.
> 

Added the line "UserAuthFile=Xauthority.temp" to section [Display] in
/etc/sddm.conf again.

Result:

sudo sudo xauth -n -f /run/sddm/* list
hostname/unix:0  MIT-MAGIC-COOKIE-1  035594cc2ff0ee82aaa4d016b3ef5a60

sudo xauth -n -f /home/michael/Xauthority.temp
hostname.localdomain/unix:0  MIT-MAGIC-COOKIE-1 
035594cc2ff0ee82aaa4d016b3ef5a60

So NOW they'vc identicalvalues, but the /run/sddm/* entry does NOT have a FQDN.
No idea why, does this have an impact? 

Remark: As said, with the UserAuthFile=Xauthority.temp line I'm NOT able to
login anymore, hangs, even with the previously working old sddm pkg. Have to
revert after testing for this issue.