There is regression in KDE Plasma 5.5.3 (or maybe 5.5.2):
If you move mouse to panel and wait for tooltip or preview of application's window, then move mouse to empty place, then again move over same object of panel, KDE Plasma crash/restarts.
Workaround: K > Settings > Configure Desktop (or System Settings) > Desktop Behavior > (Workspace), uncheck "Show Informational Tips"