(Help) Port gets blocked despite open firewall

Situation:

- LMTools is running on a virtual server in the internal network and is usually connected through the internal network

- LMTools should be accessable through the public ip in the internet

Network:

Internetgateway: LANCOM883; Ports 27000-27009 are forwarded to the external address of the firewall

Firewall: UTM 9; Rule "Any->27000-27009->Server" is permitted; DNAT "Any->27000-27009->External Address to Server" is active

Windows Firewall: deactivated on both pcs and Server

Problem:

When trying to access the Server with the PC located in the internal network, The firewall logs the following traffic:

Firewallrule: PC-IP->WAN-IP: granted

Nat-Rule: WAN-IP->Firewall-External-IP

Firewallrule: WAN-IP->Server-IP: granted

LMTools is accessable

​

When trying to access the Server with a PC located outside the internal network.

Nat-Rule: Outside-WAN-IP->Firewall-External-IP

Firewallrule: Outside-WAN-IP->Server-IP: granted

Firewall logs the same traffic until the PC times out.

LMTools is NOT accessable

​

Same Situation if I try to access the Server from the guest Vlan with just internet access.

Firewallrule: PC-IP->WAN-IP: granted

Nat-Rule: WAN-IP->Firewall-External-IP

Firewallrule: WAN-IP->Server-IP: granted

LMTools is NOT accessable

​

Can't find out why the latter doesn't work. If I disable either the FW-rule or the DNAT I also can't connect from the internal network. So the traffic has to pass the FW