Comment # 3 on bug 1231007 from Peter Sütterlin

permissions are fine:
woodstock:~% l ~/.config/kwinrulesrc
-rw------- 1 pit users 1457 Sep 26 15:17 /home/pit/.config/kwinrulesrc

interestingly, the entry says

[1]
Description=Vivaldi settings
clientmachine=woodstock.pitnet
clientmachinematch=0
desktop=4
desktops=66a5393b-0706-488c-8693-af039e23c6ad
desktopsrule=3
title=Start Page
titlematch=0
wmclass=vivaldi-snapshot
wmclasscomplete=true
wmclassmatch=2

But when I go to system settings, it shows 
Virtual Desktop           Apply Initially         All Desktops