What else might cause the "Can't open" - Error?

Hi. I wanted to test a very simple mean reversion strategy on my OANDA demo account via the V1 API. Most of the time (maybe 90%) when I get the buy/sell signal, Zorro is unable to open the trade, e.g. [BCO/USD::S] 10:22 Can't open 94@53.35. I think it is not a problem of missing price quotes or lacking of liquidity as mentioned in the manual. I also browsed through the forum but couldn't find anything. What other reasons are possibly causing this error? Thanks!

You can see that in the log.

Thank you. I checked it but the log-file only contains the "Can't open" messages without any hints why. And there are no other error messages.