UPDATE:
Well, I called in last Friday, but they replied it took longer than two days for them to have a look at, dispite what they promised me. They called Saturday, but I was at work at the time, so I called today (Monday's closed due to holiday season) and they knew what was wrong! So I drove to the shop and they told me that most likely the northbridge chip is fried, due to a not-working northbridge cooling fan. I also had an explanation as to why it worked before (with the non-working cooling fan), because I had SD-RAM installed at the time. When I installed the DDR-RAM, it had to work a lot harder and it didn't get the cooling it needed, so it fried.
The costs for this were 20 euros, but I could get a 20 euro discount on a new motherboard. The cheapest one they had was some Gigabyte one and it costs 84 euros! Minus the 20 euros would be 60 euros, but that is still waaay to much. I asked if they sold any Asus boards, because the fried board was an Asus as well... 120 euros!!! And that was the basic model. I don't know any model names, but it is waaay to expensive.
In a quest on finding a new motherboard, I fired up the internet and I came by a shop called Computerland which had a sale of old motherboards, including an
Asus A7M266 for 29 euros! New, but obselete, because non of the Athlon XP CPU's fit on that board, but that doesn't matter, since I have an AMD Thunderbird, which should fit fine. Shipping costs 11 euros, so that would be a total of 40 euros, which is a decent price.
The new mobo should arrive in a few days, so maybe this week, the server can be put online again!