Thanks man, it worked like a charm for me;
1. I used the Siemens MPM (Mobile Phone Manager) to put the px65v5 midlet on my phone. After ~800 s it returned the IMEI, ESN and HASH.
2. I recharged my phone and removed my SIM-card.
3. For this step I used an english version of Papua Utils, v079c to be exact.
4. First I got a little confused at this step. You need to transform the ESN number from 12345678 to 78563412 (if you use px65v5, px75v1 does this for you), and leave out the "-" (the minus signs the midlet returns).
5. After I got all three codes right, it calculated the SKEY and BOOTKEY just fine.
6. This step I did a little different than as described here. When I tried pressing "Enter SKEY" while booting Papua Utils returned something like "cannot open communication on com 3" (yes, com 3 was the port my phone was attached to). However, when I let it boot to the screen which read "Please insert SIM", and pressed "Enter SKEY" then, it worked just fine, and Papua Utils told me I was in Factory Mode.
7. This step worked also perfectly. Clicking the "Freia.log" button opened a dialogue asking if you want to save some *.log file. After pressing "cancel" another dialogue fired up, asking if I wanted to write directly to my phone. After pressing "Yes" Papua Utils told me it wrote 4 blocks, and everything went fine.
After this operation, my phone still worked. I switched my SIM-card with my brothers, who has a different provider, and it worked, so my phone is now officially unlocked.
Big thanks again man.
P.S.: A nice trick to find out which Com port your phone is attached to: In the "Settings" tab of Papua Utils, press "Information". If the right port is selected, a list of information about your phone is returned by Papua Utils. If not, an error is returned. Change port and repead procedure until you recieve information about your phone. This way I traced the right com port.
P.S.2: For both the data transfer (putting px65v5 on my phone) and the actual unlocking I used some "China-made"-USB-cable.