Uninstalled. I couldn't figure out what was wrong, but in my LM 21.1 / Claws 4.0.0 version it busted my setup after installing and setting notifications: when I chose the system tray notification it froze the top navigation bar in my Claws and removed the navigation buttons (the usual ones for shrinking and closing any window). When I restarted Claws, all plugins had been unloaded. I reloaded and picked a different notification setting, but it all happened again. Once I uninstalled this plugin, all was fine. No clue what's up, but something is. Only good I installed the plugins consecutively!