So a fellow guild member and I have been having the exact same connection issue and it's beginning to drive me mad.
So we seem to disconnect at the end of every dungeon that we run. Get to the end, loot the boss, DISCONNECT. Very rarely does it happen in the middle except for VDSA when entering a new arena or if there's a load screen involved. We both get told that we "cannot connect to the server" when it does this.
I have been through port forwarding to ensure that it wasn't a port forwarding issue. I just can't find the reason for this. We're both with Virgin Media for our Internet. Any suggestions or solutions would be AMAZING. We waste about 15 minutes of time in every vDSA run just because of this.
Dark Flare is the Beginning, Radiant is the End. Hail the Light Bringers!