Danke.
Yes, FXCM has spreads (they call it markups in pips) and rollover. Commission is really zero. I just downloaded via API the AssetFix.csv (asset.csv) and this was confirmed. No commission. And also in the Broker reports no commision, which is the case for many neobrokers novadays as well.
I will check the values of the trades based on this updated AssetFix file in the next days.
For me it was strange that the issue was found for short positions only. The strange thing was that the profit was almost the expected value, but only with an opposite sign.On the other hand it has shown a gain, even when I had a real loss???
I have no explanation for this so far. I will do as you have recommended. Calculate the .log of the trade data from start to the end. I let you know the result of my investigation.