Need help. Firewall XG WAN Link down at Night

Hello community,

first sorry for my bad english. I have installed Firewall XG Home Version on my QNAP. All work fine. On my QNAP (TS-251) i have 2 ETH. One Port (ETH0) is going to my FritzBox 7490. The other port (ETH1) is going into my Switch and the rest of my IT.

ETH0 had IP: 192.168.0.x 

ETH1 had IP: 192.168.2.x

Setup: --> INTERNET-->FritzBox-->LAN1-->SOPHOS(WAN)-->SOPHOS(LAN)-->SWITCH

all work fine. But at night / morning the WAN Port (ETH0) is down. No Internet Connection is possible. Where can i find some informations about this Problem?

When i restart SOPHOS, all works fine. But nights the same Problem accours