i had the same issue on a blueonyx 5208 with centos 6.6 
IPv6?
Yep had to disable it, it was driving me crazy
But you activate IPv6 after installation ?, for now has not happened to me that case.
Regards
Happy Weekend
Just to say I had the same issue with a fresh nethserver install and disabling ipv6 (as above) fixed it.
I didn’t explicitly enable IPv6 but I have the red interface set to DHCP and my ISP provides an IPv6 address by default, so it was enabled.