"May be the reason it work for greater then 25cm is "THE LOW SPEED 300bps"."

Probably, but the fact that it works does not prove that it works.

Chips are made with tolerances and the fact that this works may be just sheer luck. I particularily do not like the fact that you are using connectors that makes things worse.

When I stated 25 cm I assumed it was on the same board, or at least the same bus.
I strongly recommend that you put 232 converters on the boards, at least if this is for commercial purposes. Without the 232 converters you are looking at CMOS drive which equates to virtually no noise immunity (high impedance).

Have fun, but it is more fun to do things right.

Erik