"Order Can Only Have One Flag Outside RTH or AllowPreOpen" (when submitting a GTC_ext order through the API) This only happens to one person. When I try to place exactly the same order with the same parameters on exactly the same TWS version, the error doesn't occur. Anyone have a clue how to fix this?
IB recently changed something on the API side related to flags on API submitted orders. You need to update your IB Gateway or TWS to the latest version. I'm still having issues and cannot log into mobile TWS, but Desktop TWS and TWS gateway work fine. Mobile IB Key also works fine. Sounds like your issue is different, but you could try deleting all your open orders and updating TWS to the latest.
According to here: https://interactivebrokers.github.io/tws-api/message_codes.html "This error is deprecated". Probably you are still using an older version on this account. Unfortunately does that table not give any further clarification as to why you are receiving that error code&message.