[GT-I9001] USB connection not working

Hi guys.

I want to flash my SGS+, but Win7 doesn't want to recognize it via USB...
Devmgmt.msc lists it as...
- "Unknown Device"
- Error Code 43: "Windows has stopped this device because it has reported problems"
- Hardware-ID: "USB\UNKNOWN"

I've read quite a lot about this topic, but nothing has worked so far:
- no connected devices in Samsung Kies
- no connected devices using "adb.exe devices"
- deinstalled Kies and installed the drivers directly (Samsung USB Mobile Composite Device / MODEM Mobile Composite Device / Mass Storage...)
- switching modes (Mass Storage / Kies / Media Player; neither of these modes works)
- tried both PDA and MODEM via *#7284# (USB I2C Mode Control)
- tried 2 other USB cables on various ports (USB 2.0 of course)
- Kies Air works, but doesn't allow me to flash it

Booting into Download and Recovery Mode works, so the phone isn't bricked I guess. Still no USB connection in these modes with Odin or ADB, though. Whatever I do, Windows always lists it as Unknown and my SGS+ does nothing but charge...
I tried connecting a friend's smartphone (Galaxy S II) and I had no problems using both Kies and mass storage, so it's gotta be an issue with my SGS+...

When rebooting from Recovery Mode, although I only hit reboot now without installing anything, I get the same errors written in yellow everytime:

Code:

---

>>> status = 0 <<<
-- Updating application...
E: failed to mount: /preload (Invalid Argument)
E: install_application_for_customer:Can't mount /preload

-- Applying Multi-CSC
Installing Multi-CSC...
E: Cant access to '/system/csc/DTM/system/'.
Successfully applied multi-CSC.
status  1...
Rebooting...

---

Maybe this is related? Any idea if there are any Recovery Mode Fixes for this or how to debug it?

My Phone:
Samsung Galaxy S Plus, Stock ROM, rooted via Root-i9001-Signed.zip
Model number: GT-I9001
Firmware version: 2.3.5
Baseband version: I9001BOKP1
Kernel version: 2.6.35.7-perf / root@DELL133 #1
Build number: GINGERBREAD.BOKP4