Internet connection issue

[Tattoofreak]

Now this issue is driving me crazy:
I'm just not able to connect my QNAP NAS TS-453s to the internet anymore and I was just not able to find something wrong.
My setup is the following:

Two LAN-ports, both are connected to a physical switch.
LAN1 is on VLAN 1.
LAN2 is on VLAN 100 and connected internally to QNAPs virtual switch.
Both LAN ports are getting their IP-address from a DHCP-Server, served by the physical router (Ubiquiti USG).
Virtualization Station is installed and keeps a virtual machine (windows server 2016 as domain controller and DNS server).
The QNAP is member of the domain.

No matter what I try, i'm just not able to get an internet connection.
All my other devices (physical desktops with windows10 and linux mint machines) in my network are connecting without problems to the internet.

The NTP server is set up correctly, all devices are getting their time from the same NTP server.
DNS is also set up correctly.
IP-adresses are not deployed twice, I already checked that.

The strange thing is that every other device in my network is working correctly. Only the NAS is getting no internet, even when I switch the default gateway from one LAN port to the other.
I'm really not getting this.
Help from a QNAP pro would be very appreciated.