Samsung GT-i9305 destroyed after Downgrade? Odin - sboot.bin FAIL

Hello at all!

First: Sorry for my english. I'm from Germany,but I hope you understand me. I these Forum I think you get the best tips,so I'm here. :)

My Device: A Samsung Galaxy S3 / GT-i9305

I got a big,big problem with my Phone. I wanted to root it at the beginning ... it doesn't match,because I had Android 4.4.4 and I don't find a solution,do root it with thissystem. So I wanted to downgrade it to a lower version. After ring this my device don't start, it hangs at the Samsung Logo (bootloop) and there is no recovery mode. Download Mode works just very rarely - I don't know the reason why.
If i get it to the Download Mode, I tried to flash in Odin Android 4.4.4 , but all the time there is the same message. You see here:
<ID:0/003> Added!!
<OSM> Enter CS for MD5..
<OSM> Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> I9305XXBMD5_I9305DTMBMD1_I9305XXBMD5_HOME.tar.md5 is valid.
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/003> Odin v.3 engine (ID:3)..
<ID:0/003> File analysis..
<ID:0/003> SetupConnection..
<ID:0/003> Initialzation..
<ID:0/003> Get PIT for mapping..
<ID:0/003> Firmware update start..
<ID:0/003> SingleDownload.
<ID:0/003> tz.img
<ID:0/003> NAND Write Start!!
<ID:0/003> sboot.bin
<ID:0/003> FAIL!
<ID:0/003>
<ID:0/003> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)


It shows every time the same error with 'sboot.bin'.
Now I bought a USB Jig to get at least to download mode.

Is there any solution or Idea to run my good,old S3?
I would be very thankful if someone could help me with my problem.
Sorry for my english, but I hope you know what I want to say :)

Nice Greets,Postmaschine