I realize this thread is a little stale and I don't know if you found a resolution or not. In my case I found my RT-AC66U_b1 (same hw as 68 I think) running Merlin (any version so far; currently running 384.5) doesn't seem to like SSH over wifi -- wired works fine. If I'm connected directly to the AP over wireless I get the same SSH problems you described. If I connect through another AP (which then connects via wired) or through a host on a wired port I don't have issues. Changing to allow SSH over WAN did not work either.