You may try to use
timeframe = bt.TimeFrame.Daily
I'm new here, I don't know if this could work, but, emmmm, have a try, maybe it's good.
@steffen Is there a quantitative trading platform support the new python3 tws api? I'm looking for something could backtest and trade using Interactive Brokers, just like the BackTrader, but the ibpy is too old to use.