I just got the above mentioned switch and have connect three QNAP TS431XeUs with DACs. The first one I connect to SFP1. When I connect the second to SFP2 the second port began to switch between link up and link down. Do I plugged it in to SFP3 and the link stayed up. Same with the third in SFP2 so plugged it in SFP4. SFP2 to SFP4 are configured for an untagged VLAN with the same PVID (128). SFP1 and ether1 Support VLAN 128 tagged. The first QNAP ist configured for using tagged VLAN on the 10 GB-Link.