EDIT4: It works win xp -> N900 ok with Microsoft BT stack. However I had the same problem with N900 -> win xp. I see the same error in syslog which means that win BT side doesn't support COM port over BT (RFCOMM). When I configure "Incoming COM port" for BT on win xp side I'm able to send file also from N900 -> win xp I'll try also the BT stack you're using (WIDCOMM - under Win XP)
EDIT5: N900 -> win xp with WIDCOMM is working for me. However I don't find a way to try OPP win xp -> N900. obexFTP win xp -> N900 using "send to" function has the problem that it tries put it into root directory for obexFTP where it's forbidden (in stock too) However merlin1991 has the same problem as you so maybe he can provide me access to some test case.