Fortunately, I was able to fix the problem myself.
I went through the following steps: As I already had removed the battery pack, I removed the charger as well and finally removed the BIOS-battery. Then I shorted the pins of the bios battery (+/-) for a few seconds with some tweezers. Afterwards I reinstalled the main battery pack and connected it up. Finally, I was able to power the notebook back on.
[image|2831206]
[image|2831205]