I know that this is an old thread - but I just unboxed a C5210 yesterday and had this same experience. Do you remember what support said?
I fumbled around for 30 minutes or so trying different cables, using a small unmanaged switch instead of a direct connection, disabling my wireless and virtual VPN NIC's, etc. Then, out of nowhere I could reach the server having just let it sit for 15 minutes or so.
After going through the wizard and setting the initial two IP addresses, I lost connection to the box (to be expected). But then I couldnt' reconnect to it on it's new address. So I fumbled with that for a while, eventually hooking up my laptop again and going through the same thing of not being able to reach it with an IP from the same subnet.
Then I just happened to try the 192.168.10.0/24 network and the 192.168.10.1 management IP, and it worked! So it seems that when it should have changed the IP address, it didn't for some reason. However - I did force-reboot the box at some point too when I was getting desperate and worried. Not sure if that is what happened to the OP in this thread.
I should be in good shape now, because I don't think I will have problems connecting in (post-wizard) and setting things the way I want. But I thought I would share my experience in case others run into this.