I solved this problem. The problem wasn't in the firmware. I don't really understand why it works now. I don't know why it helped but the problem was in my router. I set static-DHCP for my PB, and now it works. I just gave a fixed IP address to MAC address of PB912. It even works with the latest firmware.
I noticed this problem when I restarted my router, I could connect to wi-fi. Unfortunately, when I disconnected PB from wi-fi, I couldn't connect to wi-fi again. Then I restarted the router again, and I could connect to wi-fi again. So I added PB to static DHCP table, and it worked. Guys, can you explain me why it helped? It's kind of a mystery for me.