Nested virtualization networking problems - L2 not able to reach local network

[reinis]

I have set up a Proxmox server that has an Ubuntu CT running apache webserver and a Ubuntu VM hosting GNS3, and the GNS3 has a Ubuntu Server virtualized using qemu. In the GNS3 has a bridge to the Ubuntu VM, so all the Ubuntu Server is able to access the internet, receive its ip from a DHCP server and so on, however it is failing to cURL the webserver and any other service hosted on Proxmox. The webserver is accessible from any other client on the local network, so there is not an issue with that. And the weird thing is that I am able to ping the Ubuntu Server from the webserver and also the other way around. The same issue is when trying to connect to the Proxmox instance.

I can confirm that when performing curl command I see the traffic reaching the Uubuntu VM from the Ubuntu Server in GNS3, however no reply is heard back:

13:34:24.516279 IP 10.0.0.192.58596 > 10.0.0.115.http: Flags [S], seq 1572958192, win 64240, options [mss 1460,sackOK,TS val 582325567 ecr 0,nop,wscale 7], length 0
13:34:24.516531 IP 10.0.0.115.http > 10.0.0.192.58596: Flags [S.], seq 1402438565, ack 1572958193, win 65160, options [mss 1460,sackOK,TS val 2185326685 ecr 582325567,nop,wscale 7], length 0

The current setup:
Proxmox > Ubuntu CT with Apache on port 80
Proxmox > Ubuntu 18.04 VM > Ubuntu Server (using qemu on GNS3)

/etc/network/interfcaces on Proxmox:

auto lo
iface lo inet loopback

iface enp2s0 inet manual

auto vmbr0
iface vmbr0 inet static
        address 10.0.0.101/24
        gateway 10.0.0.1
        bridge-ports enp2s0
        bridge-stp off
        bridge-fd 0

/etc/netplan/00-installer-config.yaml on Ubuntu VM:

network:
  ethernets:
    ens18:
      addresses:
      - 10.0.0.120/24
      gateway4: 10.0.0.1
      nameservers:
        addresses:
        - 8.8.8.8
        - 8.8.4.4
        search: []
  version: 2

and /etc/netplan/50-cloud-init.yaml from Ubuntu Server inside of GNS3

network:
  ethernets:
    ens3:
      dhcp4: true
      match:
        macaddress: 0c:64:b7:a5:00:00
      set-name: ens3
  version: 2

Any idea why is this issue is happening? I have research this topic a bit and all I could come up with was some keywords from other posts, like enabling promiscuous mode, but that seams to apply only for people using VMware, or someone did mention setting the bridge_ageing to 0, but that didn't help either.