CARP virtual IP high packetloss

[wk]

I have some issues running my firewall in HA. My network is attached, as well as my CARP setup.

As soon as I start my backup firewall, I experiance a package loss of 10-50% on all virtual IPs of the WAN side. I don't have that issue on the LAN side. There is no package loss accessing the firewalls on their unshared IPs. I tried to test some random vhid, to rule out interference on this by my ISP, but it doesn't work.
As far as I can tell, there is now split brain between both firewalls, the main one is the master, and the backup is slave and the simulated failover works. The switch between the firewalls and the ISP Router (Adtran NetVanta 4660) is an unmanaged TP-Link. I used the same switch on the LAN side, again with no issues.

Thank you!