KDE desktop crash

I am migrating an old, SSE only, computer from 13.2 to Tumbleweed. Now the system boots normally until graphical login screen. When I enter the pasword, the KDE desktop does not start but I get a window with a notice that kwin_x11 crashed.
In logs there is a notice that something crashed due to lack of SSE2 extension, repeated several times.
Is SSE2 CPU needed for TW (or KDE 5)? Everything other seems to work.
Runing kwin_x11 in gdb show forking four times, then a SSE2 notice and ABORTING message. How to check what exactly is crashing?

Wojciech

Yes.

Actually, Qt5 requires SSE2 for certain things, QML in particular (which is heavily used by Plasma).