As a beginner i made the experience that it is much easier to get positive backtest results with indices and metals than when trading FX. Any reason for that?

I wouldn't say that it's easier to trade. It's just a different instrumentent but basically the same. It can be bullish and bearish. I guess you will find out, as I did over time, that testing results can be very dedulded and there are many factors which are influencing the results. So, as you are at the very beginning, I would say your script just fits better indices & metals but that can change any time. Take time. Don't hurry. Get rich slowly.