Cannot cast Spotify with Chromecast v2

So I've been banging my head against the wall for the last couples of days trying to cast spotify to my second gen chromecast but I can't make it work.

I'm using UTM 9 Home Edition. I can cast any other apps with no issues (Google Music, Plex, Youtube) except spotify. When on my android phone, I can see the chromecast and select it. The loading screen pops-up for a couple of second on my TV then it goes right back to the chromecast wallpapers (backdrops).

I'm having the same problem as this guy:

https://productforums.google.com/forum/#!topic/chromecast/0LPBfVYwRS0

but he never mentioned how he fixed it :(

Here's what I've tried so far:

• Check the firewall logs and permit both incoming and outgoing connection to the chromecast
• Disable IPS, AV, Country Blocking, Web Filtering
• Add an exception to Web Filtering and select every checkboxes for the chromecast
• Enable Ping and ICMP to and from the gateway 

It's weird because it's seems something is dropping the packets because when I go in the overview of Web Protection (Where there is a couple of pie charts) I see that the #1 host with dropped packets is my chromecast (50%). I just can't seems to find what's dropping them.

Here's some more information about my setup:

UTM 9 Home virtualized

2 interfaces (WAN & LAN)

Asus AP plugged in a gigabit switch

Any ideas what might cause this ?

Thanks !