strange ucom (uplcom) error

Dear experts,

I have two USB-RS232 Adaptors, both with PL2303 chipset. One is working the 
other one not (I hate to say it but both are working under win).

The not working (more expensive) one gets recognized as ucom0 and I have 
ucom0, also I can receive signal but not transmit.

When I connect the (more expensive) adopter to a serial console (FreeBSD 
comconsole) I can see all kernel messages but I can't type any charecter when 
I see the login. If I change to the cheaper adopter everything is working 
fine. I compared 'stty -a -f /dev/ucom0' and both adaptors show the same 
settings.

I'm using tip (ucom0:dv=/dev/ucom0:br#115200:pa=none:) on RELENG_5 (from this 
weekend)

Here is usbdevfs output and the corresponding dmesg line for both adapters, 
the 2nd (ATEN) is the working one (cheaper one), the first the more expensive 
not working one!

port 1 addr 2: full speed, power 100 mA, config 1, USB-Serial 
Controller(0x2303), Prolific Technology Inc.(0x067b), rev 3.00
__dmesg:
ucom0: Prolific Technology Inc. USB-Serial Controller, rev 1.10/3.00, addr 2

port 1 addr 2: full speed, power 100 mA, config 1, PL2303 Serial adapter 
(ATEN/IOGEAR UC232A)(0x2303), Prolific Technology(0x067b), rev 2.02
__dmesg:
ucom0: Prolific Technology PL2303 Serial adapter (ATEN/IOGEAR UC232A), rev 
1.10/2.02, addr 2

Any help highly appreciated!

-Harry