Alexa says "<device> isn't responding" but the Homematic IP device group still turns on / off like it should

I’m running openHAB 3.0.1 via openHABian 1.6.3 on a Raspi 2 Model B Rev 1.1 connected to a Homematic CCU2, and via openHAB Cloud connected to my Echo Dot.

At first controlling my lights worked perfectly, but since a couple of days, when switching lights on or off, Alexa responds with " isn’t responding. Please check it’s network connection and power supply." but the device still switches on or off flawlessly despite the error message.

Anyone had this behavior before?

Yes I get this with the google home sometimes. Its like the assistant is waiting for a response and doesn’t get one in time so it thinks it is not working.

You may need to have Alexa rediscover openHAB devices (from the Alexa page on amaz*n.com)

Although I still cannot reproduce the problem (it only occurs randomly once every once of times times), I was able to narrow it down a bit more, and have reasons to believe that it has to do with Homematic IP / the CCU in one way or another: While the problem does not occur when switching individual lamps on and of, it only occurs when switching the openHAB Group Item (combining three lights) on or off. The reason why I believe that it has to do with the way the group interacts with my Homematic IP CCU2 is that, already when moving the slider in openHAB (-> where Alexa is definitely not yet in play), the behavior of the three lamps and the slider itself is not yet fully consistent (sometimes a lamp is missed, sometimes the slider is not updated properly).

As a quick fix I just created the group in Alexa itself (instead of “in openHAB” as above), which now works like a charm.

A root-cause based fix would probably involve to understand the transfer of command to and from the Homematic CCU a bit better. E.g. I vaguely remember from the CCU-manual that “three comments being sent at the same time to the CCU” can cause problems (something with overlaying signals), which is exactly what I believe openHAB is doing with a group of three lights.