OpenVPN with Multi-WAN

Hello,

I've problems to config Multi-WAN and OpenVPN.

OpenVPN connection works only if port 80/81 run on the same interface as OpenVPN.

I've tryed following configuration:
eth2 (primary) WAN slow DSL with Static-IP (I'll use for OpenVPN)
eth3 (backup) WAN LTE with Dynamic-IP (I'll use for surfing)

Destination Port Rules:
Port 80 (http) eth3
Port 81 (https) eth3
Port 1194 (OpenVPN) eth2

With this configuration, OpenVPN connection works only somethimes and only for a short time. If port 80/81 is on eth2 OpenVPN works stable.

What can i do?

Thanks in advance
Robert