"Waiting for available socket" error when using "switch dashboard on item change"

On my CM3-Home board (Raspberry PI3 equivalent) with OH 2.4 (a brand new installation not an upgrade), I’m trying to use the “switch dashboard on item change” function. I’ve created an HABpanel item and defined an entry for it on the classic UI through sitemap:

Frame label=HABpanel 
	{
        Text label="HABpanel" 
		{
			Selection item=HABpanelSwitch label="HABpanel [%s]" mappings=["Giorno"="Zona Giorno", "Notte"="Zona Notte", "Audio1"="Volumio 1", "Audio2"="Volumio 2", "Audio3"="Volumio 3", "Condizionatore"="Condizionatore", "Energia"="Energia", "Meteo"="Meteo", "Esterno"="Esterno", "mbar-CO2"="mbar-CO2", "Interno"="Interno", "CronoTermostato"="CronoTermostato", "EnergyG"="EnergyG", "Internet"="Internet", "CPU"="CPU"]
		}
	}

Everything works fine for some switching, let’s say 5 or a bit more, then the behaviour of the HABpanel becomes extremely slow. No errors on log and no heavy CPU load as you can see on attached image. It’s slow in every function related to web server: directly loading a new panel (also apart from the classic UI switches) or UI or log window until a long waiting or a full reboot. Chrome says “waiting for a socket available”

Any idea?

thank you in advance
Guido