Wow! I hooked up the Mac Mini to the monitor I used for the insallation and... voilà it works! The resolution is 1680 x 1050.

@Kronos
Next thing I will try, is to delete those monitor files (found them) and check that out with the other monitor (just out of curiosity).

Anyway, you can consider the problem solved! Fantastic! Thanks so much guys! :)

The adventure goes on....

Cheers