HmIP-BDT-Dimmer is (sometimes) unreachable for ioBroker

[arniworx]

hi!
i'm using a HmIP-BDT Dimmer connected to a Homematic CCU2.
every now and then ioBroker tells me the device is "UNREACH". at the same time, i can control the device directly from the CCU2. does anyone know, this behaviour?
i think it's a device-setting at the CCU2 - but which one?
thanks in advance.
arni