In the meantime I was looking for ideas on how to solve the issue (turning on and taking out the battery, taking out the battery for a longer time, etc.) but nothing helped. I saw a post (http://forums.androidcentral.com/sam...e-wi-fi-4.html), that seemed to be plausible and I thought I would give it a try.
Yesterday I rooted my phone (with Odin and CF-Auto-Root-jflte-jfltexx-gti9505.tar.md5), loaded the newest SuperSU-app and was able to actually see, what the problem was. Logcat showed this, when turning wifi on:
03-05 21:52:36.347 I/WifiManager(1215): setWifiEnabled : true
03-05 21:52:36.347 I/WifiService(830): setWifiEnabled: true pid=1215, uid=10266
03-05 21:52:36.357 E/WifiHW (830): ##################### set firmware type 0 #####################
03-05 21:52:36.357 I/WifiHW (830): module is murata FEM1
03-05 21:52:36.357 E/WifiHW (830): ==========[WIFI] Station firmware load ===========
03-05 21:52:36.357 E/WifiHW (830): ==========[WIFI] MURATA FEM1 MODULE ===========
03-05 21:52:57.207 E/WifiHW (830): return of insmod : ret = -1, No such device
03-05 21:52:57.207 E/WifiStateMachine(830): Failed to load driver
03-05 21:52:57.247 E/WifiStateMachine(830): sendErrorBroadcast code:10
03-05 21:52:57.247 D/STATUSBAR-NetworkController(1215): onReceive() - RSSI_CHANGED_ACTION, WIFI_STATE, NETWORK_STATE
03-05 21:52:57.257 E/WifiController(830): Wi-Fi driver is unstable. Received CMD_STATEMACHINE_RESET
03-05 21:52:57.267 I/WFMonitorService(4771): WIFI_STATE_UNKNOWN
This happened also after deleting or renaming the wpa_supplicant.conf (both in system/etc/wifi and in data/misc/wifi) and restarting the phone. I also tried deleting or renaming the whole wifi-folder (in both directories), without success. Renaming the file worked, but it wasn't recreated upon restart, which seemed pretty strange. Upon renaming the wifi-folders only one was recreated (the one under data/), but the .conf-file was still missing. I continued getting the error message cited above.
I searched for the "device not found" issue and found the following thread (http://forum.xda-developers.com/show...php?p=50535766), but this didn't help either. Even setting permissions for the files and folders that I thought would be important didn't help.
Today I wiped my phone clean in recovery and tried flashing the stock firmware I9505XXUFNB8_I9505OXAFNB8_I9505XXUFNB8_HOME.tar.md 5 on, but wifi was the same. I even tried flashing only the modem with files I found here: http://d-h.st/users/DjeMBeY/?fld_id=15780#files. Something changed, because when I turned wifi on it turned itself off right away, so it didn't last in the dark green status for a few seconds as it used to. I also recall seeing this error message, as opposed to the one above and it was running slow with different other errors (no sound, apps crashing, etc.):
E/WifiHW ( 793): return of insmod : ret = -1, Exec format error
I really don't know what to do with it anymore. Since bluetooth is working I don't think it is a hardware issue and I should be able to resolve it somehow. Since I rooted it I don't know if I can use the warranty and I didn't want to send it in for 2 weeks.
I would be very happy about any ideas on how to resolve this issue. I hope there enough information. Thanks in advance!