It looks that michaaa is right and that a single addition to systray creates the problem.
To test the issue I have added milou to systray and diffed with a working profile, shit happens in file .kde/share/config/plasma-desktop-appletsrc
Here is the diff of the offending lines:
81c81
< geometry=172,3,1026,32
---
> geometry=172,3,998,32
90c90
< geometry=1202,3,32,32
---
> geometry=1174,3,32,32
99c99
< geometry=1238,3,162,32
---
> geometry=1210,3,190,32
105a106,112
> ShowApplicationStatus=true
> ShowCommunications=true
> ShowHardware=true
> ShowSystemServices=true
> ShowUnknown=true
> alwaysShown=
> hidden=
112a120,122
> [Containments][1][Applets][7][Configuration][Applets][1][Shortcuts]
> global=
>
121a132,134
> [Containments][1][Applets][7][Configuration][Applets][13][Shortcuts]
> global=
>
127a141,143
> [Containments][1][Applets][7][Configuration][Applets][14][Shortcuts]
> global=
>
133a150,152
> [Containments][1][Applets][7][Configuration][Applets][15][Shortcuts]
> global=
>
137a157,165
> zvalue=0
>
> [Containments][1][Applets][7][Configuration][Applets][16][Shortcuts]
> global=
>
> [Containments][1][Applets][7][Configuration][Applets][17]
> geometry=0,0,24,24
> immutability=1
> plugin=org.kde.milou
Now the question is, provided that there must be some upstream bug, what to do?
THX