Fresh install of Kubuntu 20.04.1 (Plasma 5.18.5), plus installed Kdenlive from Muon (didn't came by default, and now I know why). When starting Kdenlive, Plasma crashed, asking a minute later to login again. After this event, I lost all animations and all window borders (except for the top bar) for all apps. Checked animations and themes (kde and gtk2+3), and all is set as it should. Will reinstall and replace Kdenlive with Pitivi. Sorry.

Adding the Kdenlive PPA solved the app issue with a new Kdenlive version (newer than the one downloadable from KDE apps): https://kdenlive.org/en/download/
Unfortunately, the damages to Plasma persisted.