Its stupid but
where the hell is hide SSID option in 5.21?
it does not work for nv2
ups dont know that.
Wireless - Interfaces - Wlan1 (or whatever wlan interface do you have) - Wireless tab - on the bottom of page you have a check box Hide SSID
Try
Wireless protocol=nv2
and find it 
http://wiki.mikrotik.com/wiki/Wireless_Matrix#Wireless_Features_Matrix
Is not possible to use ‘Hide SSID’ option with ‘NV2’.
HTH,