RFXTRX dimmable lights are not available occasionnally

I have several lights controlled via 433MHz dimmers (KAKU), RFXTRX. I control them via Home assistant and luckily also via the KAKU controller (ICS2000). Occasionally some lights appear unavailable in Home assistant. I see the following message: “Deze entiteit is niet meer beschikbaar vanuit de rfxtrx integratie. Wanneer de entiteit niet meer in gebruik is, kan deze vanuit de instellingen verwijderd worden.”
(This entity is no longer available, when the entity is no longer in use it can be removed from the settings)
When I restart Home Assistant Core, ALL KAKU dimmers are unavailable.
I need to change the light intensity with my KAKU controller in order to have them available in Home Assistant again.
Is this a configuration issue? Please advise