After that it was imposible to connect to any WiFi router (tried different ones that were working OK with other devices). I did a factory reset and still no WIFI connection possible. I always got "Network disabled. Poor connection" even with full signal and right by the router. So, I flashed the original ICS stock ROM and every was OK again, WiFi worked perfectly. I tried one more time the JB OTA update and again and ended up with NO WiFi.
I have also tried every trick I´ve found on Internet (like disabling wifi power saving, etc, etc) nothing worked.
Anyone knows how to solve it? I will go back to ICS but that´s not the idea when buying a top smartphone, is it?
Rooted, and flashed with stock ROM I9300UBELL1 = Kernel 3.0.31-767276 se.infra@SEI-75#1 SMP PREEMPT Mon Dec 31 13:32:27 KST 2012 ... it seems it is an older ROM but Sammobile.com listed it as a more recently released named as I9300UBELL6_I9300ANCELL1_ANC while the previous one was named I9300UBELL1 ... the last digit is the revision number, isn´t it?
Its not the RIL/modem becuase before that I tried flashing a different RIL firmware and WiFi didn´t work either.
Hope it helps for those left without WIFI.