I had the same problem yesterday. I ended up doing this:
- Uninstall Duokan
- Uninstall hacks (USBNetwork, Fonts, ScreenSavers etc.)
- Uninstall Jailbreak using latest uninstaller
- Backup all my books (to Calibre)
- Do a factory reset
- Update to 3.4 (finally worked!)
- Recover books from Calibre
I was hoping that I can avoid factory reset, but unfortunately nothing else worked for me.
One thing different from yours is the fact that during my fight I have installed the latest jailbreak again (previously I had an old jailbreak which I uninstalled to no avail). Maybe try to install latest JB and one of the hack to confirm that JB is there, and then uninstall it and redo the factory reset.
Last edited by krzyk; 10-10-2012 at 10:45 AM.