Fire TV Home Theater not working with Zyxel NWA130BE

Hey,

Network Setup:
- GS1900-8HP Switch as central switch unit, IGMP Snooping enabled
- 2x NWA130BE in Nebula cloud mode
- Fritz!Box 7590 (only for DECT)
- Opnsense Firewall as router/firewall

I'm using a FireTV Stick Max 2nd Gen together with two Echo Studio in Home Theather Mode. When pairing this devices the FireTV opens a separate wifi ap so the Echo Studio connecting directly to the FireTV Stick.
Before I have bought the NWA130BE all devices were connected at Fritzbox Wifi (5Ghz, signal is strong enough) and working all together.
After I installed the Zyxel AP and disable Fritzbox Wifi, it is working generally (devices are connecting to one Zyxel AP) but every minute I have got audio stuttering for 3 seconds and then it works again for one minute.

I have checked multiple settings and situations. If I change back to Fritzbox everything is fine, with Zyxel not :(

What I can see: There is some kind of multicast running on this connection (I think with this they are checking if the connection is working).
If the devices are connection to the Fritzbox I can't see much multicast traffic, only sometimes the igmpv3 reports. If they are connected to the Zyxel I see many multicast on other ports. But IGMP Snooping is enabled on the switch (it is the same switch to which the Fritzbox is connected to).

Maybe you can help me in this case. Thanks!