Chromecast 2 issues with UTM-9

Hi All,

I've been happily using Chromecast for a while and decided to buy the latest version for another TV.

Unfortunately, although the ''old' chromecast works perfectly, I cannot get the chromecast 2 to work.

I can connect to the chromecast 2  and run setup. When I select my WiFi and enter the access code, I can see the chromecast message that it is connecting to my WiFi, but that's all. The app times out and then I can retry.

When I re-run the setup on my old chromecast, it all works fine (same WiFi network, same AP).

I've created a static DCHP entry for the chromecast 2, gave if full access in the FW rules and added exceptions in the ATP and IP. It still doesn't work.
If I check the FW logs, I can see the Chromecast 2 communicate with google,nothing blocked. ATP en IP logs remain silent.

My UTM is placed behind the modem provided by my internet provider. If I place my AP directly behind the modem (so no UTM), it all works fine.

I tried to be smart and configured the Chromecast 2 in the working setup, then placed the AP behind the UTM again. Now the Chromecast 2 is initializing (it connects to google and the default backdrops are appearing), but still no way to connect to it from within my home network (communication failure reports when I try).

I'm puzzled :)  Any suggestions as where to look next?

Karl-Heinz