I have a 802.11n nv2 link on two RB411AH running RouterOS 5.0rc9. The signal level and data rates are high (-50 dB, 243-270 Mbps). When the traffic is high the link disconnects from time to time (after a few minutes when there is 100 Mbps of traffic, after half an hour when the traffic is lower). The logfile says "lost connection, control frame timeout".
I found on forum that nv2 may cause such problems. But when I switched to nstreme (setting wireless protocol in WinBox) the link didn't work at all showing strange signal levels (above zero).
I also found a suggestion to set default-authentication=no and add MAC address to connect/access list, but that didn't help.