[TYPO3] multiple domain login problem

[Bertalan Fodor]

Hi,

I have a one-tree-multiple-domain installation. I can only login to the 
'master' site, however, both frontends appear correctly.
  is missing. I mean on http://www.firstdomain.com/typo3 I can log in, 
but on http://www.seconddomain.com/typo3 I can't, the login screen 
appears, then but after giving my credentials, the "Login-error or 
session timed-out No user logged in! Sorry, I can't proceed then! (You 
must have cookies enabled!)" message appears. I checked the cookies, and 
I can see that a cookie with PHPSESSID is placed, but the cookie with 
be_typo_user is missing.

Can you help me why this could happen, or where can I enable some 
logging to see some error message?

Thank you,

Bert