Forget my comment about the Z12.fac in Zorro 1.83.2.
The problem was my AssetsFix.csv. More precisely: The RollLong and RollShort values for XAG/USD were off by a factor of 50! That wonders me because my assets file was created by Zorro after a test with connection to the MT4 bridge. It seems the Assets.csv generator in Zorro doesn't respect that according to http://manual.zorro-project.com/afaq.htm the rollover fee should be per contract! LotAmount is 50 for XAG/USD in my case. If I manually divide RollLong and RollShort by 50 and write that into the AssetsFix.csv then it is fine.