in the wireless client registration table we had in 2.9 the datarate column,
separated with /. For example 12Mbit/36Mbit.

Now in 3.0rc5 in some cases SP appears in this column just like this:
12Mbit-SP/36MBit. What doeas this SP mean? This appeears mostly just after
the client has connected and then disappears. Mostly when this SP is in the data rate, the client cannot communicate with the AP...

SP - Short Preamble.

btw, don't make mutiple topics.
http://forum.mikrotik.com/viewtopic.php?f=1&t=18694