Problem solved. I didn't mention that the server running Filezilla is Win2003 DC with RRAS enabled. It seems that RRAS has some kind of FTP helper/filter too, and it must be playing with IP addresses. I tried a few different FTP softwares, and the result on this particular machine was the same.
I didn't want to waste any more time on this, so I just moved FTP server to another machine without RRAS. No more problems, passive works as it should.