Thanks a lot for your answer!

In the meantime the same thing happended again with a different trade:

(GBP/USD:CX:L) Long 434@1.35891 Entry limit
!position closed by reversal
Error 075 (GBP/USD:CX:L) - can't open 434@0.0 Stop 0.075288 at 00:22:28

The pending trade above change the lot size of trade 181:
Zorro shows the lot size to be 879 lots:
Trade ID Lots Entry time
[GBP/USD:CT:S00181] 181 879 12-16 12:03

Oanda shows a new lot size of 445 Lots for ID 181.

I have set NFA=0 and Hedge=0 as stated in the manual. Therefore no hedging is allowed.

Why does Zorro create a Long trade when Short trades for GBP/USD are still open? The Hedge=0 shoud have prevented this, right?