I have had the cooling fan replaced plus the relay. (The relay was melted!)
The fan worked fine for a few days but now it stopped working again.
The relay was again melted.
replaced teh realy again today and it works again.
The fan is protected by a 40amp fuse which is still ok.
I measuared the amps today with the fan running and it varie sbetween 12.1 and 12.9. Personally I think this is very high for a running fan.
Any suggestions what the problem could be?

Your problem may be a seized fan motor bearing. Not totally locked up, but close. Even a new fan can be defective. My '90 Escort fan draws 4.5 amps running....
Good luck!!