When you simultaneously go long and short in the same transaction with Interactive Brokers, they report the transaction as only one line in the trade history. This will cause Negative Shares and incorrect Profit/Loss when imported.
To fix this, please undo the last import by clicking on Edit, then UNDO. Now go to Account, then Edit and check "Short/Long Convert". This will break up any trades in which you go simultaneously long and short into two trades. Take note of the pop-up, and click Yes. Click OK to save the account settings. Now re-import.
If using Edit, UNDO is no longer practical because of too many edits, you can click on ALL Trades to temporarily close the Negative Shares error screen, then click on Edit, then Force Match Trades.
Please be sure to verify your open positions. Force Match is a very powerful function that can radically change your trades - see: Force Match Trades
Before your next import, go to Account, then Edit on the IB account tab and check "Short/Long Convert", then click OK.