What kind of "investigation" did you complete? What does it mean?
It means, I tried to debug php code to find out - why, I'm being redirected to the home auth screen
each time I try to enter my Admin's login/pass at bb_admin page.
This investigation leads me to the following: $_POST array is empty.
I'm not sure, it's Lighttpd-related problem.
As I mentioned before - test script (for POSTs) works well,
just in the same directory (means - with the same Server/Php configuration)
What else could it be?
Toggling Safe Mode ON/OFF did not help.