UNIFI AP – NAT – Only one AP can be connected to controller at a time

[ VPS ] — [ NET ] —- [ NAT ] — [ UNIFI ACCESSPOINTS ]

If You can connect only one AP at time to newly created site on Your Unifi VM controller, and AP from NAT network connect to VPS f.g.

check if You have:
net.ipv4.tcp_tw_recycle = 1
in /etc/sysctl.conf

and revert it to
net.ipv4.tcp_tw_recycle = 0

Magic 😉