unable to upload to Sparkfun Pro Micro

I just got 2 Sparkfun Pro Micro and am unable to upload any sketches to them. I have seen that others have had similar problems with the Leonardo, but none of the tricks they reported helped (e.g. trying direct USB vs. USB hubs, repeatedly installing drivers etc.). I have tried it both under Windows 7 and Windows XP (dual boot on the same PC). Can anybody help?

Here is the verbose log:

Forcing reset using 1200bps open/close on port COM21
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, COM21, } => {}
PORTS {COM6, COM7, COM21, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, } => {}
PORTS {COM6, COM7, } / {COM6, COM7, COM21, } => {COM21, }
Found Leonardo upload port: COM21
C:\Program Files (x86)\Arduino\hardware/tools/avr/bin/avrdude -CC:\Program Files (x86)\Arduino\hardware/tools/avr/etc/avrdude.conf -v -v -v -v -patmega32u4 -cavr109 -P\.\COM21 -b57600 -D -Uflash:w:C:\Users\Ralf\AppData\Local\Temp\build4100269646280052002.tmp\Blink.cpp.hex:i

avrdude: Version 5.11, compiled on Sep 2 2011 at 19:38:36
Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
Copyright (c) 2007-2009 Joerg Wunsch

System wide configuration file is "C:\Program Files (x86)\Arduino\hardware/tools/avr/etc/avrdude.conf"

Using Port : \.\COM21
Using Programmer : avr109
Overriding Baud Rate : 57600
AVR Part : ATmega32U4
Chip Erase delay : 9000 us
PAGEL : PD7
BS2 : PA0
RESET disposition : dedicated
RETRY pulse : SCK
serial program mode : yes
parallel program mode : yes
Timeout : 200
StabDelay : 100
CmdexeDelay : 25
SyncLoops : 32
ByteDelay : 0
PollIndex : 3
PollValue : 0x53
Memory Detail :

Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


eeprom 65 10 8 0 no 1024 8 0 9000 9000 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


flash 65 6 128 0 yes 32768 128 256 4500 4500 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


lfuse 0 0 0 0 no 1 0 0 9000 9000 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


hfuse 0 0 0 0 no 1 0 0 9000 9000 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


efuse 0 0 0 0 no 1 0 0 9000 9000 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


lock 0 0 0 0 no 1 0 0 9000 9000 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


calibration 0 0 0 0 no 1 0 0 0 0 0x00 0x00
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack


signature 0 0 0 0 no 3 0 0 0 0 0x00 0x00

Programmer Type : butterfly
Description : Atmel AppNote AVR109 Boot Loader

Connecting to programmer: .avrdude: Send: . [1b]
avrdude: Send: S [53]
avrdude: Recv:

avrdude: Recv:
avrdude: Send: V [56]
avrdude: Recv:
avrdude: Send: v [76]
avrdude: Recv:
avrdude: Recv:
avrdude: Send: p [70]
avrdude: Recv:
Found programmer: Id = "aÿ("; type = ?
Software Version = E. ; Hardware Version = ð.
avrdude: Send: a [61]
avrdude: Recv:
avrdude: Send: b [62]
avrdude: Recv:
avrdude: error: buffered memory access not supported. Maybe it isn't
a butterfly/AVR109 but a AVR910 device?

The miracle has happened! Why? I don't know.

What I did was that I took the first Pro Micro off the USB cable, connected the second one and I am told that the driver (which I had installed a few times before) was not installed. I installed the driver yet another time and - whow - it worked. The best thing is that when I switched back to the first Pro Micro it still worked. To me it looks a bit as if my computer has finally decided to stop bugging me. I just hope that it will still work tomorrow...