VM1 cannot ping VM2 on different VMBR

[sITs]

Hi,

my problem is, that VM1 cannot ping VM2 on a different VMBR.

We've got 3 Proxmox-servers running as a cluster. Each server hat its own /29-subnet
(e.g. IPs 1.2.3.1/29, 1.2.3.9/29, 1.2.3.16/29) and some failover-IPs for
critical VMs (e.g. IPs 2.3.4.1, 2.3.4.2), so there VMs can migrate between servers.

Our current setup is like this:
- VMs on the server-subnet (e.g. IP 1.2.3.2) use VMBR0
- VMs that use the failover-IPs use an internal subnet (192.168.0.0/24 mapped to the failover-IPs) on VMBR1

The problem is, that a VM inside a subnet (on VMBR0) cannot reach a VM on a Failover-IP.
There not even a response to a ping.

Situation for VMs on VMBR0:
+ they can ping each other
+ they can ping all 3 Proxmox-servers
+ they can ping the outside e.g. google.com
- they cannot ping VMs on VMBR1

Situation for VMs on VMBR1:
+ they can ping each other
+ they can ping all 3 Proxmox-servers
+ they can ping the outside e.g. google.com
- they cannot ping VMs on VMBR0

Situation for the 3 Proxmox-servers:
+ they can ping each other
+ they can ping the outside e.g. google.com
+ they CAN ping VMs on VMBR0
+ they CAN ping VMs on VMBR1

Here's the interfaces-file of one of the servers on the cluster:

# Loopback device:
auto  lo
iface lo inet loopback

# 1Gbit (external)
auto eno1
iface eno1 inet static
    address       4.3.2.6            # ServerIP
    netmask       255.255.255.255
    pointopoint   4.3.2.2
    gateway       4.3.2.2

# Failover-IPs
    up ip addr add 2.3.4.1/32 dev eno1
    up ip addr add 2.3.4.2/32 dev eno1

# Failover-IP Routing
    post-up iptables -t nat -A PREROUTING  -d 2.3.4.1          -j DNAT --to-destination   192.168.0.162
    post-up iptables -t nat -A POSTROUTING -s 192.168.0.162    -j SNAT --to-source        2.3.4.1
    post-up iptables -t nat -A PREROUTING  -d 2.3.4.2          -j DNAT --to-destination   192.168.0.163
    post-up iptables -t nat -A POSTROUTING -s 192.168.0.163    -j SNAT --to-source        2.3.4.2


# 1Gbit (internal)
auto eno2
iface eno2 inet manual
    mtu 9000


# 10Gbit (internal)
auto enp5s0
iface enp5s0 inet manual
    mtu 9000


#Ceph Bond
auto bond0
iface bond0 inet static
    bond-slaves enp5s0 eno2
    bond-mode active-backup
    bond-miimon 100
    address 10.10.10.1
    netmask 255.255.255.0
    network 10.10.10.0
    broadcast 0.0.0.255
    mtu 9000


# /29 Subnet 1.2.3.1/29
auto vmbr0
iface vmbr0 inet static
    address       1.2.3.1
    netmask       255.255.255.248
    gateway       4.3.2.6
    bridge_ports  none
    bridge_stp    off
    bridge_fd     0


# /24 Internal Subnet 192.168.0.0/24
auto vmbr1
iface vmbr1 inet static
    address       192.168.0.1
    netmask       255.255.255.0
    bridge_ports  none
    bridge_stp    off
    bridge_fd     0

Any ideas how I can let the VMs reach each other?