Is it prohibited to the 5GHz radio, or does it also happen on the 2.4GHz radio?
Do all accesspoints have the same behaviour?
Does the logging give you any clue?
Sure the VLAN section is correct? I’m missing the frame-types=admit-only-untagged-and-priority-tagged on the /interface brdige port.
But my knowledge on VLAN is very limited (though I know I prefer using VLAN’s for all networks).
Perhaps (also) have a look at this great topic:
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1