flashed it with
RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM
and
RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM

using flasher 3.5

Didn't help at all.
Problem persists.

I have even removed the back housing which has the antenna on it and replaced it with my previous n900. it didn't help.
I placed the "new/used" phone's housing on mine and it was working.

So the problem is not on the "outer" antenna and not a software problem too.
Do you suppose it might be a problem with the board wifi connector I show with green arrow? Can this be fixed somehow?