With KWin master as of today the configuration of the Translucency Effect changed. It used to have translucency values in the area [0.0, 1.0] which got changed to [0, 100]. If you rebuild and just restart KWin it might be that you do not see any windows at all, because the windows are completely translucent.
To circumvent this problem run the KConfig Update application in $KDEDIR/lib/kconf_update_bin/kwin_update_settings_410 or just log out and log in again.
Users not running master are of course not affected by the change, the KConfig Update handles it correctly on first login after update.