@joomlanewbie: great reply but not the one I`m looking for . instead of masking small bugs I prefer to solve them.
anyway I think I`ve found reply in this thread :
http://www.joomlahacks.com/component/option,com_smf/Itemid,94/topic,362.0

gotta try it...

As I have similar login issues I thought I`ll post here.
Login in Mambo works wine - I`m being logged in and out from both mambo and smf. The problem is with smf login. When I try to log in through smf I get a javascript aplication message "Please complete the user name and password fields". The intresting part is that logging out from smf works fine - I`m being logged out from both smf and mambo. Any ideas how to fix it?