[Desire Android Development] Narfx:
Same here. First run of superuser -> fc. After that, it works. Logcat:

I/ActivityManager( 153): Starting activity: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.noshufou.android.su/.Su }
I/ActivityManager( 153): Start proc com.noshufou.android.su for activity com.noshufou.android.su/.Su: pid=1478 uid=10039 gids={1015}
W/ResourceType( 153): Skipping entry 0x7f040007 in package table 0 because it is not complex!
W/ResourceType( 153): Skipping entry 0x7f040008 in package table 0 because it is not complex!
D/dalvikvm( 1478): GC_EXTERNAL_ALLOC freed 1251 objects / 87256 bytes in 40ms
D/Su ( 1478): First run for version 21
I/global ( 1478): Default buffer size used in BufferedReader constructor. It would be better to be explicit if an 8k-char buffer is required.
D/Su ( 1478): su version: 2.3.1-ef
I/ActivityManager( 153): Displayed activity com.noshufou.android.su/.Su: 521 ms (total 521 ms)
E/Su.Updater( 1478): Problem downloading file
E/Su.Updater( 1478): java.net.SocketException: Permission denied
E/Su.Updater( 1478):	at org.apache.harmony.luni.platform.OSNetworkSystem.createStreamSocketImpl(Native Method)
E/Su.Updater( 1478):	at org.apache.harmony.luni.platform.OSNetworkSystem.createStreamSocket(OSNetworkSystem.java:186)
E/Su.Updater( 1478):	at org.apache.harmony.luni.net.PlainSocketImpl.create(PlainSocketImpl.java:265)
E/Su.Updater( 1478):	at java.net.Socket.checkClosedAndCreate(Socket.java:873)
E/Su.Updater( 1478):	at java.net.Socket.connect(Socket.java:1020)
E/Su.Updater( 1478):	at org.apache.harmony.luni.internal.net.www.protocol.http.HttpConnection.<init>(HttpConnection.java:62)
E/Su.Updater( 1478):	at org.apache.harmony.luni.internal.net.www.protocol.http.HttpConnectionPool.get(HttpConnectionPool.java:88)
E/Su.Updater( 1478):	at org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnectionImpl.getHTTPConnection(HttpURLConnectionImpl.java:927)
E/Su.Updater( 1478):	at org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:909)
E/Su.Updater( 1478):	at org.apache.harmony.luni.internal.net.www.protocol.http.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:1152)
E/Su.Updater( 1478):	at com.noshufou.android.su.Updater$DownloadFileTask.doInBackground(Updater.java:136)
E/Su.Updater( 1478):	at com.noshufou.android.su.Updater$DownloadFileTask.doInBackground(Updater.java:126)
E/Su.Updater( 1478):	at android.os.AsyncTask$2.call(AsyncTask.java:185)
E/Su.Updater( 1478):	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
E/Su.Updater( 1478):	at java.util.concurrent.FutureTask.run(FutureTask.java:137)
E/Su.Updater( 1478):	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
E/Su.Updater( 1478):	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
E/Su.Updater( 1478):	at java.lang.Thread.run(Thread.java:1096)
D/AndroidRuntime( 1478): Shutting down VM
W/dalvikvm( 1478): threadid=1: thread exiting with uncaught exception (group=0x4001d7e8)
E/AndroidRuntime( 1478): FATAL EXCEPTION: main
E/AndroidRuntime( 1478): java.lang.NullPointerException
E/AndroidRuntime( 1478):	at com.noshufou.android.su.Updater$DownloadFileTask.onPostExecute(Updater.java:167)
E/AndroidRuntime( 1478):	at com.noshufou.android.su.Updater$DownloadFileTask.onPostExecute(Updater.java:126)
E/AndroidRuntime( 1478):	at android.os.AsyncTask.finish(AsyncTask.java:417)
E/AndroidRuntime( 1478):	at android.os.AsyncTask.access$300(AsyncTask.java:127)
E/AndroidRuntime( 1478):	at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
E/AndroidRuntime( 1478):	at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 1478):	at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime( 1478):	at android.app.ActivityThread.main(ActivityThread.java:4627)
E/AndroidRuntime( 1478):	at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 1478):	at java.lang.reflect.Method.invoke(Method.java:521)
E/AndroidRuntime( 1478):	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
E/AndroidRuntime( 1478):	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
E/AndroidRuntime( 1478):	at dalvik.system.NativeStart.main(Native Method)
W/ActivityManager( 153): Force finishing activity com.noshufou.android.su/.Su
D/skia ( 153): purging 201K from font cache [13 entries]
W/ActivityManager( 153): Activity pause timeout for HistoryRecord{4479edf0 com.noshufou.android.su/.Su}
I/ActivityManager( 153): No longer want com.tat.livewallpaper.aurora (pid 794): hidden #16
I/Process ( 1478): Sending signal. PID: 1478 SIG: 9
I/ActivityManager( 153): Process com.noshufou.android.su (pid 1478) has died.

(edit) Looking at the log it seems to be Su.Updater which causes the FC. I repeated the FC by checking for updates in options.