Okay I got it sorted out. After hours of headache and trying to find a way to fix it by reading so many posts here, the problem turned out to be because of my hosting company. I rang them and explained the situation, then the guy told me sth about a difference in the php versions which the server supports compared with the one 4.2.4 requres. If someone know more about it, please share some more light on it, cause I was embarrassed to ask the guy for a deeper explanation.
