By accident I found the following:
After rearming my glaive via an f-key macro and accidentally hitting Lesser Defensive Stance, it worked. My Fighter was in LDS. Trying again the same with GDS. Also the usual server message "You must have a shield equipped" was not appearing.
So a probable stupid question: Is this now a normal feature or a bug (I remembwr Ariki posting something about a "nasty plate bug" he corrected)?
It's probably a bug. But honestly, if fighters can parry with pole arms, I'd think that they should be able defensive stance with them as well (though not as well as with a shield, obv)