Comment 14 for bug 1504626

I have upgradeded from Kubuntu 15.04 to 15.10 yesteday and got the same error.

For me this looks like a shell script error. E.g.:

HOME=$(some | invalid | command | sequence)
KWALLET_CONF="/$HOME/.config/kwalletd5rc" # results into ""//.config/kwalletd5rc"

I also made an investigation about which process spawns the message:
elvenfighter ~ $ ps -eo pid,ppid,args | grep -i kwallet
 1196 1174 [kwalletd] <defunct>
 1197 1174 /usr/bin/kwalletd5 --pam-login 17 21
 1788 1197 /usr/bin/kdialog --title kwalletd5 --msgbox Configuration file "//.config/kwalletd5rc" not writable. Please contact your system administrator.
 1793 1 /usr/bin/kwalletd --pam-login 17 20
 2489 2442 grep --color=auto -i kwallet

elvenfighter ~ $ ps -o pid,ppid,args -p 1174
  PID PPID COMMAND
 1174 1022 /usr/lib/x86_64-linux-gnu/sddm/sddm-helper --socket /tmp/sddm-auth950d1696-1757-4438-91c7-c06607824e4e --id 1 --start /usr/