I was using a dnsmasq DHCP/DNS server but that version broke due to an software update where it was running. So I changed towards the DHCP and DNS of Ubiquity Edgemax Lite

The issue I had was that local domainnames (FQDN) could not be looked up by its name but only through IP. In the old situation where not the router was the DNS server it was working but in the new environment not. I changed the DNS server to the correct local IP for the local hostnames, but no success.

Than I found a hint in a forum

open a SSH session towards the Edgemax

configure
show service dns

forwarding {
cache-size 400
listen-on eth1
listen-on eth2
name-server 10.1.1.1
name-server 8.8.8.8
name-server 8.8.4.4
system
}

Above is the non working environment for VPN (for all other situations it was ok)

the local name-server was defined but it was probably not available through L2TP VPN

The solution was to add

options listen-address=10.1.1.1 with this command:

configure
set service dns forwarding options "listen-address=x.x.x.x"
commit
save

where x.x.x.x is the IP of the router. It should be the IP address that PPTP/L2TP is returning to the clients for DNS server