in deinem Log steht
Vtarget : 0.0 V
Varef : 0.0 V
haben die denn vorher Spannung selber generiert oder hast du vielleicht einfach nur eine schlechte Versorgung?

Setting jumpers:
J1 Power target
Supply target with 5V (USB voltage). Be careful with this option, the
circuit isn't protected against short circuit!
J2 Jumper for firmware upgrade (not self-upgradable)
Set this jumper for flashing the ATMega(4)8 of USBasp with another working
programmer.
J3 SCK option
If the target clock is lower than 1,5 MHz, you have to set this jumper.
Then SCK is scaled down from 375 kHz to about 8 kHz.